I've been using it for about a month now, and I've found it's really changing my development process. The line by line code coverage analysis is almost a replacement for booting up the debugger, and it's surprising how much time is saved just by being able to continue coding while your tests are running.
I've also found it very stable. It's never slowed down VS, or crashed hard enough to stop me from working. The only issues I've had with it have been the result of my unique development requirements. I work with SharePoint2010 alot, and NCrunch has trouble running tests that directly interact with sharepoint, or for objects that are already in the GAC. However, neither of these are show stoppers, and it works great for the rest of my code.