He's definitely much better this season than last. It's made a pretty big difference for United, especially away from home.
Plenty of limitations still. He doesn't press properly, although that'd be less of a problem alongside Parker than it is with Scholes or Giggs. He does almost nothing at either end of the pitch, so by playing him instead of Gerrard or Lampard you're definitely cutting a few goals out of the side. He makes huge mistakes in important matches, which isn't exactly a quality you want in the Euros. I suspect players love having him on their side because his simple passing game makes everyone else's job much easier, but I don't think the opposition particularly minds facing him either. The biggest point in his favour is that when he doesn't play, England are crying out for a player that can play a few simple passes in midfield and keep the game under control. Wilshere and Cleverley can both do that as well as bringing a bit of energy in defence and creativity going forward, but since they're crocked, I think it has to be Carrick.
Also, unlike most of his generation, he's done well for England before.