PS. From what I understand it is a one click upgrade to 2.5 from 1.7.x - given that 2.5 is a stable release I wouldn't expect to see massive changes from the latest 1.7.x if they're aiming for stability.
Yup, it's 1 click upgrade, but they are are making framework changes which do cause adverse affects for extensions. Right off the bat for example CB and VM2 both don't work on J2.5 (we've an SVN build of 2.5 to test on). We're reviewing the cause of this of course and looking for a fix.