I'm currently running OpenELEC v2 Beta 5 and it is flawless. My wife loves it because it is brain-dead simple. I use the Android app for remote control, but have also used a generic Media Center remote an IR receiver. On my project list is to get a Bluetooth remote. I hate having to aim.
The hardware I use is a Zotac HD-ID11 mini-PC. For my use it is almost, but not quite, perfect.
OpenELEC has a tailored distribution just for this type of box. It is optimized for the nVidia ION-2 chipset and under 2 Gb in size.
I don't use a hard drive at all. The system is loaded onto a 4 Gb SD card for total silence, less heat and less power.
The box is mounted on a little VESA bracket that screws into the back of the TV. Perfect fit and totally out of sight. The only cables are HDMI to the TV, power and USB/IR adapter.
My only complaint is their choice of wireless chipset. Included as a mini-PCIe card is an Azurewave combo card which is 2.4 GHz only. That is, 802.11 b/g/n + Bluetooth 3.0.
My experience has been not good in playing HD video (h.264) or audio (Ogg-FLAC) on 2.4 GHz. I have too many things in that frequency in my house and it causes buffering and lag.
So today I opened the box and replaced the Azurewave with an Intel 6230 combo card for $23. Essentially the same specs but with 5.0 GHz a/n support as well.
That solves my problem. There is no lag or delay in playback of HD video or audio using this wireless connection. It isn't as snappy as when it is connected to the GigE wired link, but that is really overkill and this means one less cable and freedom to rearrange furniture.
Everything in this little unit works right out of the box with Linux. The Intel wireless card also just works. No drivers to download, no nothing.
Now to convince it to stream from my Amazon Prime account or Netflix. I think there is an XBMC plug-in for those.