Agree with some of the above comments, he was a bit of a fairy and primadonna but he was a great player more often than not when he was on the pitch and fit.
I'd say that he was by far the most productive (in terms of actual direct influence...goals, assists etc) of all of Sven's signings. Too bad that he couldn't stay fit and/or on good terms with Hughes as a fit Petrov probably would have saved him his job.
;-)