I got a Freezer 64 Pro from Newegg this week and installed it about an hour ago on my 64 X2 3800+. I decided to go with the included thermal pad since it's already there. Installation was easy and so far it is running 2C cooler than my previous HSF (a CNPS7000/Al-Cu) and noticeably quieter.
I got this along with an opty 165. When i was running at 2.44 and after running 2 instances of prime95, the temps went up to 36C from 31C. My conclusion, i would leave the stuff on, as u can see from my temps, they are extremely cool