I've been using an HTPC for media playback since the late 90's. Most of my experiences were fairly negative, as most of my HTPCs had many problems, but recently all that changed when I got the HP Digital Entertainment Center z560. I have to say that it is the best HTPC I have come across overall.
My first few HTPCs were custom built, and I had many driver issues (especially with the ATI All-In-Wonder cards) so I decided to go with a prebuilt unit. Things I liked on the HP are the following:
1. XP MCE is extremely reliable
2. I didn't have any problems with the HP drivers
3. No bloatware
4. Great for multitasking, i.e. you can burn a CD, download pictures from a camera, and play a video at the same time without any problems
5. Connectivity such as HDMI, multichannel audio out, optical, and coax integrate very well in a HT environment
So - I guess I've found my first "good" HTPC and I'm very happy with it. Now how many people have HTPC's on this forum?