A terrible upgrade process that took forever and resulted in VMs that ran so slow I couldn't even install Parallels tools without timing out. Support tickets and forum posts failed to get a useful response.
I switched to VMWare 2 (free after competitive upgrade) and will never go back.
For those people making posts about why run Windows in a VM, I would respond that I think all software developers should run each of their projects in a separate VM (as I do) to prevent contamination of the OS's. I run 3-5 VMs at once, all running Visual Studio, with different projects in each. The freedom of this also allows me to use the best tool to host the VMs, surf the internet, read my mail and do everything else, except write software for MS based clients, in Mac OS X.