I went ahead and got one of these. Immediately installed linux XBMC on it.
My inital impressions are good, with a few huge caveats. First, I don't care about a ton of "high-end" features - my collection is almost all compressed rips with no HD audio, and I've been using a Neo 550 with almost no issues since they launched - so keep all that in mind. Compared to the average poster here, I'm pretty easy to please.
First off, the remote is total jank. I have a harmony, so no big deal to me. The box gets quite warm during use, and I'm a bit concerned about overheating but I have nothing to back up that worry yet.
I immediately installed the Jan. 29th linux XBMC image (the latest one on Pivos' site), which was painless, except for having to dig up a SDcard (my unit didn't offer to install from a USB device).
My initial impressions are pretty good. The unit is very responsive and the UI is faster than my NeoTV550. During library updates it slows down significantly but it's still quite usable. The audio choices were a little confusing at first, but that's because the install automatically assumes it's going to be putting out audio over HDMI, so instead of selecting audio output you simply choose what your receiver can handle. I didn't check about HD audio passthrough, but as mentioned I don't really care about that. Otherwise it's pretty much XMBC and all that it entails. I installed a couple "official" add-ons with no issue.
No problems with playback on any of my formats, including a bluray .iso structure. No buffering issues that I can see, but my media collection is all compressed formats (no 1:1 BR rips).
My big issue right now is the box just doesn't seem stable. Over the course of the night I had at least 5 hardlocks/reboots. Two were from trying to use Airplay, which worked great for about a song then quit working, and the others were during media playback. Additionally, I saw what looked like some playback stutter - every ~5 minutes or so I'd see some sort of stutter. I don't know if it's 3:2 pulldown or a display timing artifact, but it was quite noticeable.
At this point I'm on the fence. $120 for a full functioning XMBC setup is really nice, but it's still beta-quality stability. I'm going to figure out how to grab the one of the recent nightly builds and see if they've fixed the hardlocks, but if not I'd have to say it goes back.