I just installed Parallels 4 yesterday and have been playing with it since. Except for a bug where it claims that it is unregistered and can't be updated, according to the preferences, it did update and the web site shows that it is registered.
But, I have loaded two virtual machines, XP and Debian, and both work just about like they were on a native machine. XP was loaded from CD and Linux was an Internet install and both completed with no glitches. XP has no problems at all, but with Linux I haven't managed to adjust the screen resolution to optimum. Still looks fine, though.
It takes a little getting used to as regards external media. Depending on where I am at the moment, I will insert a CD or flash drive and wait for it to show up and it doesn't. Then I find that it mounted in Linux, or OSX instead of Windows. So I have to get used to disabling I/O hardware once I am through with it. Possibly I haven't found the proper way to handle mountable media yet.
As far as a load on the machine, or slowdowns, I have noticed absolutely none, even with all three active and running active programs. The CPU meters seem to be little different than when just OSX is running. However, this is a 4 core i7 IMac, so results may be different on an older machine.