The menu is different and will take some getting used to. Feels very 7D-ish. 12800 ISO shots of a pretty model on the night, transferred to a large screen were very, very good. AF is super good. The Canon bloke said there will not be a f8 AF focus fix for the 5D, just not enough processing power even though the Digic 5 is 17 times more powerful than the Digic 4.
Certainly looks well worth the upgrade to me, the senser and the AF and ISO are huge improvements.