I'm on Vista for all 4 PC's in my house BUT I loaded a new machine for a friend last week and immediately put SP3 on it. He has not experienced any negative effects in a week so far. Not exactly a long test but so far so good. I have NOT loaded it on any of my company pc's yet.
When I first installed it, I had some quirks but it wasn't on a HTPC, just my laptop. Seemed to freeze oddly when I'd right click. It only did that the first day I had it installed. After a few days, I haven't really noticed a difference. I was told there would be speed increases but nothing noticeable on my laptop.