I personally use an ATV2 right now as it suits my needs. Before I used a mini hooked to my TV - I didn't use a keyboard or mouse, and instead only used the IR remote - running Plex (or XBMC if you'd prefer that) on the mini. It played 1080p video encoded @35mbps without a problem. I only switched because the ATV2 fit better on my entertainment center and fit my needs better as I spend less time watching some of the other formats I used to and currently primarily watch video streamed from my iTunes library.
Keep in mind, I do have a lot of MKV files - to play those I either have to transcode them or use XBMC or Plex on the ATV2 (through jailbreak) but it's not as reliable for playback as the mini was.
I'm happy with my ATV2 for what I need. I a long time ago had a windows based HTPC, but since I didn't play bluray nor dvds through my HTPC, it was a waste for me as it was much larger then the mini w/ plea and didn't provide a better experience then the mini w/plex. As I moved to getting more through iTunes, the mini was just retired.
If you have a lot of files using various video containers, a HTPC (either a mini running OSX or Windows, whichever you prefer) is a better solution. If you need DVD or bluray playback functionality, windows is a better solution (DVD playback through plex or xbmc is kinda lacking in quality IMHO). If you use primarily iTunes as your source for movies, an ATV2 is a better solution.