I've been running Parallels since version 2 with great success. I use it as my .Net development environment and over the last few years have converted so many others to the Parallels/Mac way of doing Windows/.Net development that I feel I should be getting perks/gifts and/or freebies from Parallels Corporation ;-)
A month or so ago I upgraded to version 6 and ... immediately wished I hadn't. I'm currently running it on a laptop - a 2009 MacBook Pro (13"/2.53Ghz/4GB) while my MacPros at work and home are still running v5. I have seen nothing in v6 that makes me want to upgrade the install on those.
The general problem is performance - upon starting or suspending a vm (always Windows 7 Ultimate), OS X slows down, quite often freezing for a minute or two at a time. The performance of the vms themselves are fine, but for me the point of this set-up is to be able to do web-browsing, email checking etc. on the OS X side of things while doing the stuff that can only be done on Windows (Visual Studio, SQL Server tools) on Windows.
I have been using Parallels for a while so at least feel like I know what I'm doing so at the moment I am heading towards forming an opinion that its Parallels thats to blame. I've tweaked and tweaked all the vm configuration properties but to no avail. Support emails to the company have all received replies - there are no documented case of the issue you mention.
Has anyone else seen this problem and if so, have you found a fix?