I tried out JetBrains ReSharper tonight while working on a couple of things. Overall it seemed nice, but nothing special. I certainly didn't have the mind-blowing experience some others have had while using IDEA, but it is still only a “bleeding edge preview“. Since I've never used IDEA I am probably missing out on a lot of the “nice” features that have been included in ReSharper - there is a list here I will have to try out, and a good introduction at Michael Yuan's blog. The speed and stability seemed fine in the version (build 71) that I downloaded.
In a very selfish way I'm sort of glad that ReSharper is not THAT good since I have to code VB.NET at work, and JetBrains show no sign of releasing a tool for VB.NET. Now if only Microsoft will add real refactoring support to the Whidbey release of VB.NET......