I installed BitDefender 2010 Internet Security (evaluation version), but after 2 weeks I decided not to buy the licence because:
1. The update of virus definitions did not work until I went through tweaking the BitDefender firewall (it doesn't allow itself to access Internet).
2. Once the updates were done and the firewall settings modified, the CPU usage is constantly close to 100%. Even after several reboots, processes seccenter.exe and VSSERV.exe remain on the top of the most CPU-consuming processes.
3. When logging in, BitDefender apparently causes Microsoft's "Genuine Advantage" spying thing to crash.
4. Lastly a pop-up window at startup says "BitDefender services are not answering".
(Some of the names and messages are translated from French, but you get the idea)
That's enough for me. All those excellent reviews on the web were probably paid for and the reviewers never really tried the thing themselves.