Using Parallels 4.0, a desktop virtualization product, has been productive. The interface is great, performance has never been a problem - my Mac easily handles two VMs running at the same time under Parallels 4.0.

 An intermittent networking problem surfaced recently. Guest operating systems like Windows 2003 Server and Windows XP had trouble connecting to local resources when connected via Parallels Bridged networking option. Windows would sometimes report a network resource (file share, TFS connection, intranet site) inaccessible, non-existent, or unavailable. I could connect after several tries, but the connection wasn't reliable and would often drop during various operations.

Owning VMWare Fusion too, I converted the Parallels VMs that were having problems into VMWare Fusion (took about 30 minutes per VM). On booting up the VMWare Fusion VM in the exact state I left it in under Parallels, I immediately and successfully connected to all network resources. I continue to work trouble-free.

 I tried to post this on the Parallels support forum, but it is moderated and my post has not been approved yet.

I like Parallels, especially since it directly supports Mac Snow Leopard 10.6 for the host operating system (VMWare provides experimental host support for 10.6). I'll switch back once they fix this problem.