I use one of theose porter cable buffers and they work extremely well. The only downside is that when you're done, you body feels really shaky for about an hour since the buffer has crazy vibrations. You can also use sand paper with it which is a nice bonus. Buffers can get you in trouble if your not careful with them especially with dark paint. Years ago, I had a mishap with a black 911. I was close to tears as experience got the best of me. I learned to start with a side panel or an area least noticable. Even with tons of experience, I always start far away from the hood. Then do the hood last.
Spike, make sure you are using the right size counter weights. I have 2 sets (4" & 6"). When using the brush attachments, I switch to the 4" counter weights. When using the foam pads (6" & 7.5") I switch back to the 6" counter weights. Also, make sure you have the pad on there perfectly straight. That can make the buffer want to jump out of your hand!
Maybe I am just used to the Porter Cable. I have used it for 4 years (~4 to 5 hours every weekend) and I really do not have problems with vibration.
Riley