I upgraded to 2.5 yesterday, everything seems fine. But I really don't like the new interface for Widgets. In the old format all the widgets were displayed at the bottom and you could easily reuse multiple widgets by dragging them to the sidebar. This was important functionality to me because at any given time, I'll have an ad code up for a week or two, then drag it off the sidebar until the next time I need that particular code.
In 2.5 it seems like I can only have one widget at a time. Unless I add multiples to the the sidebar. So for instance I use the Text Box to add all my custom Advertising codes for contests and such. I accidentally removed one of them from the sidebar, and now I can't get it back! And I don't have this code memorized - I have it written down offline, but I hate the fact that I can't simply deactivate and reactivate it like before. This is a really bad, crappy change. And the overall Widget interface is just terrible.