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WHAT'S IN TUNE: Her feathery warble is one of the most distinctive on the season 9 roster — think Megan Joy with a better understanding of her instrument and without the consistent pitch problems. And when she chooses the right song (her semifinal ''Rhiannon,'' her Hollywood Week ''Terrified''), Didi sounds like a threat to this year's perceived front-runners.
WHAT'S OFF KEY: It'll take versatility and nerves of steel for Didi to survive 11 more weeks of wildly varied themes and intense scrutiny from the judges. If the frequently teary-eyed competitor's foray into deep-rooted soul (''Lean on Me'') is any indicator, she'll need to work on adapting disparate genres to suit her style. Plus, she cannot sing a ballad every week.
REQUEST LIST: Modest Mouse's ''Float On,'' Kate Bush's ''Wuthering Heights,'' Depeche Mode's ''Personal Jesus''
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WHAT'S IN TUNE: The only male contestant in the Top 12 who's forged a straightforward rock persona — and his gruff, slightly nasal tone is immediately identifiable, too. He's already got Simon in his corner, and he's not lacking in the looks department, either.
WHAT'S OFF KEY: Has had problems staying consistently in tune every time he's taken the Idol stage — and unless the Idol vocal coaches can help him improve his technique, he runs the risk of looking like a poor man's David Cook. Wouldn't hurt to see his song choices go a little more unexpected than Hinder or Snow Patrol, either.
REQUEST LIST: Big Country's ''In a Big Country,'' Yeah Yeah Yeah's ''Maps,'' Sheryl Crow's ''My Favorite Mistake''
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WHAT'S IN TUNE: As she showed in her audition and during Hollywood Week, Katie's got one of those big-ole voices that can take on everything from jazz standards to modern pop — and keep tweens and grandmas speed-dialing in the process.
WHAT'S OFF KEY: Lackluster semifinal showings on ''Feeling Good,'' ''Put Your Records On,'' and ''Breakaway'' can't just be blamed on conflicting advice from the judges. Katie struggles to stay in tune when she's not powering through a glory note and has failed to infuse any kind of excitement or personality into her vocals to date. Triumph over Lilly Scott in a head-to-head semifinal elimination has made her an object of rage in certain Idol circles.
REQUEST LIST: Martika's ''Toy Soldiers,'' Natalie Imbruglia's ''Torn,'' Backstreet Boys' ''I Want It That Way''
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WHAT'S IN TUNE: Three weeks running, he's delivered the smoothest, strongest vocal on the men's side — his cover of Kate Bush's ''This Woman's Work'' moved Kara to tears, and proved he's got falsetto firepower in his arsenal, too. Willingness to tackle everything from Maroon 5 to James Brown, while actually working the stage, is another strategic plus.
WHAT'S OFF KEY: Incredibly hammy stage antics during solo and group numbers gives a somewhat inauthentic slant to his performances. Needs to dial back on the pointing and winking and shameless mugging for the cameras and focus on connecting to the songs he's singing. Oh, and if Big Mike decides to break out the guitar again, it'd be nice if he actually played it instead of treating it like a jaunty prop, as he did during ''This Love.''
REQUEST LIST: Prince's ''Diamonds and Pearls,'' Jody Watley's ''Looking for a New Love,'' Wyclef Jean's ''911''
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WHAT'S IN TUNE: The haunting, crystal clear voice she displayed on ''House of the Rising Sun'' and ''Wicked Game'' is the stuff that Idols are made of. Quirky interview snippets — she's also a glassblower! she once had a Mohawk! — have made her seem more human (and less tragic) than some of her robot-like competitors.
WHAT'S OFF KEY: How quirky is too quirky? Simon is already giving Siobhan the ''dark and unlikeable'' feedback that he heaped on Allison Iraheta in season 8. And amidst a background of guitar-slinging foes, Siobhan needs to be careful her arrangements don't veer toward karaoke, as her cover of ''Think'' did in week two of the semifinals.
REQUEST LIST: Lou Reed's ''Sweet Jane,'' David Bowie's ''Life on Mars,'' Sinead O'Connor's ''No Man's Woman''
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WHAT'S IN TUNE: His witty, daring Hollywood Week rearrangement of ''Straight Up'' vaulted him to instant front-runner status — and he's got a big backlog of other crafty acoustic covers posted on YouTube.
WHAT'S OFF KEY: Subsequent semifinal performances of everything from Fallout Boy to Christina Aguilera have felt less organic — and contained more bum notes — than his landmark Paula Abdul cover. Needs to prove he's not a one-trick pony, and that he's actually got the vocal chops to compete with folks like Crystal, Siobhan, and Michael.
REQUEST LIST: Tom Petty's ''Don't Come Around Here No More,'' Nikka Costa's ''Like a Feather,'' Pebbles' ''Giving You the Benefit''
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WHAT'S IN TUNE: The appealingly lilting quaver she put to use on ''What a Wonderful World'' during Hell Week returned just in the nick of time for her solid semifinal performance of Brandi Carlile's ''The Story.'' Is it too shallow to point out her crazy-gorgeous eyes?
WHAT'S OFF KEY: Brutally pitchy take on Fleetwood Mac's ''Landslide'' turned her bandwagon into a cliff of leaping lemmings, and recent admission to Ryan Seacrest that her vision is to make ''songs that evoke emotions'' doesn't exactly point her out as an artist with a strong point of view. Plus, her attraction to leopard, gold, and pink combos reads more ''Florida retirement community'' than ''future Idol champ.''
REQUEST LIST: TLC's ''Waterfalls,'' Olivia Newton John's ''Please Mr. Please,'' Semisonic's ''Closing Time''
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WHAT'S IN TUNE: His initial semifinal performance of ''Here Comes Goodbye,'' while far from flawless, proved he can tackle a glory note with the best of 'em.
WHAT'S OFF KEY: It's hard to buy emotionally complex material from a dewy-eyed 16-year-old whose vocals contain precious little heft. Plus, Aaron's most recent song choices — ''My Girl'' and ''I'm Already There'' — were major missteps; the former sounded way too dated, the latter was far too big for Aaron's not-yet-Archuleta vocals.
REQUEST LIST: Madonna's ''The Power of Goodbye,'' Simon and Garfunkel's ''Homeward Bound,'' Jewel's ''Who Will Save Your Soul''
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WHAT'S IN TUNE: From her audition to the end of the semifinals, MamaSox has yet to hit a bum note. Her ''Give Me One Reason'' sexed-up a Tracy Chapman gem, and her Gospel-tinged cover of ''Long as I Can See the Light'' (performed one day after a scary hospital stay) was devastating in its emotional impact and quiet authority.
WHAT'S OFF KEY: While Crystal's reliance on mid-tempo classic rock jams shows she's got a strong sense of who she is as an artist, she'll need to diversify her portfolio if she wants to make it all the way to the Nokia. She'd benefit greatly from the occasional detour into up-tempo jams — or country and blues-tinged ditties. Oh, and those grim-faced shenanigans on Top 12 announcement night had best be a one-time-only occurrence; the Idol nation prefers its champs to be happy — and grateful.
REQUEST LIST: Lauryn Hill's ''Ex-Factor,'' White Stripes' ''You Don't Know What Love Is (You Just Do As You're Told),'' Guns N' Roses' ''Paradise City''
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WHAT'S IN TUNE: His cover of Bryan Adams' ''Heaven,'' while not particularly inventive, was one of the season's cleanest and most connected performances on the men's side. Casey also pulls off the rare feat of gently poking fun at the Idol machinery (check those goofy looks of panic during group performances) without seeming ungrateful or mocking.
WHAT'S OFF KEY: Choice of overused-on-Idol ''I Don't Want to Be'' showed why it's good for contestants to know their Idol history, while unremarkably competent ''You'll Think of Me'' had the distinct taste of eighth or ninth place. Unexpected song selections — inventively arranged — are his only chance to be a true contender.
REQUEST LIST: Everclear's ''I Will Buy You a New Life,'' Shania Twain's ''You're Still the One,'' the Bangles' ''In Your Room''
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WHAT'S IN TUNE: Simon continues to beat the drum that she's one of the best female vocalists in the competition; semifinal cover of Kelly Clarkson's ''Walk Away'' was solid, if slightly derivative.
WHAT'S OFF KEY: Her Top 16 week cover of ''Smile'' was an absolute disaster, raising the question of whether or not she can handle the intense pressure of Idol's big stage. And while we applaud her choice of rock ditties over big diva ballads, she's going to have to iron out her pitch problems if she wants to convert that admiration into fandom.
REQUEST LIST: Dolly Parton's ''What a Heartache,'' Ohio Players' ''Love Rollercoaster,'' Jessie James' ''Wanted''
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WHAT'S IN TUNE: After a nails-in-a-blender moment on One Republic's ''Apologize,'' his relative tunefulness on ''Come on Get Higher'' and ''Hallelujah'' was a welcome relief. Plus, Tim has handled his ''you don't deserve to be here'' critiques with relative grace and good humor.
WHAT'S OFF KEY: He's improving, sure, but even his best-rated performances are devoid of passion and originality. Plus, we haven't forgotten his mistreatment of a David Cook track during Hollywood Week.
REQUEST LIST: Amy Grant's ''I Will Remember You,'' Natasha Bedingfield's ''These Words,'' Jim Croce's ''Time in a Bottle''