I'm not here to convince anyone of anything. I'm just stating my own personal opinions.
I'd actually prefer to continue using PMS+PS3 as it's what the entire family is familiar with at this point, and requires no additional hardware changes. But currently it can only play 30% or less of my downloaded material (nearly all movies fail, most TV shows fail) and as such requires too much manual labor. Yes, it's easy to re-mux. But at the end of the day I want to sit on my couch, turn everything on, and effortlessly watch my media just like I would if I had a DVR. Not click a show, watch it fail, run back to my office, re-mux, run back to the family room, then finally watch the show. Or run to the office before watching shows, manually figure out which shows need re-mux, then re-mux them, then watch the shows. With new shows downloading daily it's a real buzz kill as I'm sure you can imagine, including for my wife when she wants to watch shows, they fail, and I'm not home. I simply want a totally hands off HTPC which is exactly what PMS+PS3 has provided until recently.
Thus for me XBMC is superior for me because it plays everything without issue in contrast to PMS+PS3 which plays about 30% or less of my downloaded material these days. I personally prefer the XBMC GUI. You can have user accounts so the kids and only see kid-friendly shares not EVERYTHING. It's smart enough that if you click a MP3 audio file while browsing videos it'll actually play the audio. That little annoyance still gets everyone in the family occasionally when we're in the audio section trying to play videos and can't figure out why it won't play. Heh. XBMC even has little check marks next to the media you've already played which is great for serialized TV shows.
I was actually hoping the cheap WD HD TV LIVE would work, but it can't see or access RARed files, thus I need to manually un-RAR everything first. PMS+PS3 had that limitation when it first came out and I lobbied for that change, which was a huge time saver. I didn't get as far as researching 3rd party firmware for WD HD TV LIVE to see if that provides RAR visibility, but I didn't quickly see where it did. Which is too bad as they're under $100 and silent. My Mom uses one in her living room, and an old XBOX running XBMC in another room.
Thankfully, there's a growing amount of affordable options for home theater media playback these days, where 2+ years ago there wasn't much out there. Good times!