Well, first off, he's always struck me as a wanker, first with the various off-field incidents (i.e - with the slightly less detestable Sreesanth), then what he does on the field. Being hard and competitive is one thing, but you can do it without crossing the line like he does (I think of Sydney and the English Ashes team of 2005 when I say this).
As a Test bowler, he reached his (admittedly incredibly high) peak in 2001, then has been slowly sliding down ever since. In fact, since 2006, he's generally struggled to be penetrative at all, with the odd 5-wicket haul here and a few good home games keeping his figures afloat. Like just about every other Indian spinner, he's very good at home and struggles overseas (I say this with the knowledge that he recently took a 6-fer against New Zealand). He is Test-class, though. He also has more variation and generates more turn than most off-spinners - plus he still gets the odd delivery to climb on the batsman.
He is also ODI-class - he doesn't take that many wickets, but he's generally economical and a very good death bowler.
The fact that he is generally given more leeway than he should received hasn't really helped his temperament or behaviour, but that is the fault of the Indian public as much as it is his. If he was an Australian, he'd be pounded like Andrew Symonds was for his recent indiscretions.