I think that Petrucci is an amazing player, he makes weird stuff sound good, but I don't think he plays anything impossible. I can play the solo in Under a Glass moon (just learnt it) and I addmitt that it was hard at first but when you play it over and over you can manage to play it quite correctly. Another point, I think that even if some of Petrucci's solos may not sound so good (don't hang me for this;) ) most of the solos are great, I can't seem to understand what does he think when he makes them.

To gather this in a nutshell. Petrucci is a great player, but he doesn't have stuff that's impossible to play (Except for Fatal Tragedy's solo). That's my opinion.