I've been a Parallels user on the Mac for around four or five years now (since the *very* early days). Each version has added features over the last, but equally each version has had its share of initial teething troubles, and the v6 upgrade is no different. Initially I had major instability, corrupted VM images (luckily I was sensible enough to backup first) and various serious problems which effectively rendered Windows 7 Pro 64-but on my iMac i7 quad core un-usable.
I had to revert back to an early v6 build, as there appeared to be problems with more recent builds, and whilst there were a few niggles at least it was stable.
In the past couple of weeks they have released a revised build which (touch wood) appears to have fixed the instability and so I am hoping that v6 will now have the stability that v5 offered, in conjunction with the promised performance improvements and the new bells & whistles (not that they are of a huge interest to me).
So, for this reason I'm giving Parallels a qualified 3 out of 5 stars, based on my mixed experiences of v6 so far and the time and effort wasted trying to get the earlier flawed builds to work. That said, if I were basing my score on the past week alone I'd be looking at four stars and if it continues to perform as it seems to be doing (i.e. stable and fast) I'd make that five stars. Sorry to be a bit vague about this but you'll understand my stance :)