i agree with boxfresh. i can't see it harming either gary or kelly's careers. certainly not gary's - take that since the reformation has been like they've been drinking from the holy grail! he'll go back to them and suddenly he'll be gary barlow of take that again. the 4 guys fit together. if mark owen confessing to being a womanising alcoholic who slept with both random nameless, faceless women while on tour, aswell as carrying on a drawn out affair with another woman for the same amount of time that he was with wife including sleeping with her the week after his daughter was born and even the week before he got married can't do anything to damage them, then i think they'll be ok! (plus there's all the charity stuff he does too, like the kilimanjaro climb or the upcoming children in need concert. stuff like that that he does that leaves x factor in the distance.)
as for kelly - she has as international career fgs. no one in america is going to be too concerned about how she's done on x factor uk (heck, i doubt anyone in america would care how she's done if she were on x factor usa!!!). as far as the uk goes - believe it or not, i think she's quite well liked outside of here. people find her annoying sometimes, but not many people scrutinise everything to the detail that it is on here. besides, she has her talent to pull her through. another hit like "when love takes over" people will be like "what? x factor? oh yeah..."
for gary and kelly, the x factor is not the highlight of their careers, and i doubt they'll stumble much away from it. for tulisa though, for MOST people, the x factor is sort of the only reason she's known, and for the people who knew her before, it was as that bratty mouthed chav that hung about with the idiot in the hat. now that she's on x factor, she's had a total make over, people are now aware of her, and she's now only planning to go for a solo career. it's provided her with that platform, so she needs it - certainly much more than anyone else. it would be a trillion times more difficult for her to establish a solo career without it. that theory is sort of backed up by the amount of slagging off her and n dubz gave it before (and in dappy's case, actually even when) she joined.
so in short (ie not another essay!), while gary and kelly MIGHT regret the decision to do it, it's probably more so a nuisance to them than anything else. a contract that needs fullfilled. it's not going to destroy them or anything. tulisa i don't think regrets it. not if she plans on using it as a platform.