So far, very minor issues.
I upgraded to v4.0 this morning. The application upgrade went without any problems.
I have upgraded my Win2K, WinXP, Redhat ELv4, Redhat ELv5, Ubuntu, OpenBSD v4.2, and Win2K Adv Server VM's so far. The only ones I had hiccups on were the XP and 2K Adv Server.
Win XP: Due to the hardware changes, I have to re-activate. It dropped out of the upgrade a couple of times so I could cancel the activate screen, it then continued to completion. No problems restarting afterwards. Activation was easy, practically just hit the 'OK' button twice and I was done.
Win2K Adv Server: Not sure what the issue was here, but it dropped out of the upgrade and I had to run the Parallels Tools file from the virtual CD drive. After that, it continued and completed the upgrade successfully. I had to go in and manually delete a SoundBlaster AWE64 driver from my device manager, but that was it.
Every other VM upgrade has been smooth. Each VM seems to be working fine for me.
MacOS 10.5.5
MacPro 2 x 2.66Ghz Dual Core Xeon
6GB DDR2 Memory