I upgraded my Gallery from 1.5 to 2.1 and man am I freaking impressed! Between WordPress and Gallery the stuff I've been working on with ASP.NET look like total crap.
The upgrade process was really easy. Rename my original gallery folder, make sure it still works, install the new gallery, import the old. The new Gallery uses MySql where as the 1.5 version only used folders to manage the albums.
Overall the experience with working with the 2.1 version is great. There is a new admin interface and just a quick look at the plug-ins shows that there is a way to upload images from Picasso! That Freaking Rocks!