Results speak for themselves: beat Ferrer and Djokovic at Roland Garros 2010, then Nadal and Federer at masters (as well as good wins over then top-10 Del Potro, Cilic and Verdasco). Had a rash of injuries after that though, and being in his 30s was never able to return to his best level. Four 250 titles and a 500 also, not a bad haul.
To some extent that 2010/11 peak was probably unsustainable 100% form, pure confidence stuff, low margin and taking risks. There was always plenty of obvious unfulfilled potential there though: excellent returns (especially of pacy serves) to compensate for his own somewhat tame serve, and he had a nice variety of options from the baseline and the net.