I am using Resharper since version 2.0 and VS2003. In those days there were major problems with the memory consumption of the tool, resulting in a slow IDE and multiple crashes. Just when I was going to remove it from my system, I made the shift to VS2005 and Resharper 2.5 (and 3.0). It couldn't have come at a better time because I never doubted the guys from Jetbrains ever again.
A while ago I installed one of the 4.0 EAP Releases (4.0.775.22). Some of my solutions not even the big ones took ages to start up and occasionally my studio froze! I quickly reverted to my old version no harm done. But a couple of days ago a Stable version was released and I decided to give it another try. My problem solution now still take a little longer to load, but I didn't suffer any freezes anymore. The support for the .NET 3.5 framework makes the loading time a necessary evil, besides you need this version if you really want to develop in VS2008 (and not be annoyed by non-compilation warnings from Resharper about the new language stuff)
You can buy a license for the currently released 3.5 and are allowed to upgrade to 4.0 once this version becomes available. So what is stopping ya?