I've used previous versions of Parallels since getting my MBP in January. Nothing dramatic, but it was at least an improvement over Virtual PC...

When the 3.0 version appeared, I tried to upgrade to it. Parallels' website repeatedly rejected my perfectly valid credit card information (on multiple cards), and refused to take my money. I wrote them about this ... a couple of days later I got a completely mechanical "thanks for your input" reply, and then a few days after that an equally mechanical "your order has been canceled."

No point in struggling further -- I picked up VMWare Fusion, paid the discounted pre-release price (no problem with the same card I tried to offer Parallels), and installed this instead. It seems completely smooth and stable, even though still beta. I see no reason to go back: Parallels made me a happy VMWare customer.