I recently upgraded to WordPress 2.5.1. Looking at the features, I was really tempted to upgrade to 2.5 but decided to wait until the .1 release. 2.5.1 fixes a lot of the bugs which were reported.
The upgrade was very smooth, and retained all my widgets, ads, layouts (which was something that used to get messed up earlier).
The new user interface is very sleek, easy to use and had a lot more features.
A noticeable new feature is the Gallery, yes it has an inbuilt photo gallery which is simple and easy to manage. I have yet to try this feature.
See the complete list of new features.