Currently I am upgrading my GPS Track Management Platform Breadcrumbs from Rails 2.3.9 to Rails 3 and finally to Rails 3.1. The upgrade went surprisingly well (I am still not finished) but after a doing the little things that Ryan is showing in his Railscast Rails 3 upgrade, my app was working again. Most of my work was upgrading the ActiveRecord finders and scopes to the new syntax.
The most cumbersome part I experince is upgrading my specs from rspec1 to rspec2. All the route_for has changed to route_to. Hopefully when that is done I can progress on to doing in-depth testing.