Firstly. Wow! I was really impressed at how easy this was to build (not quick, mind), and also with the performance of the thing itself once installed...
I followed the instructions at OpenElec (parts 1 and 2) on a fairly vanilla Ubuntu installation running on Parallels on my mac. It took a few hours to build (I went to bed, so not sure how many hours!) but had no errors whatsoever, and the instructions were perfect (I used GParted to shortcut a few of the command line partitioning steps in part 2, but that's the only thing I did differently).
Boot time the first time around is around 2 minutes, after that, it takes about a minute or so to boot. It doesn't connect the HDMI port unless it's on during boot, which is a little annoying, but not the end of the world (I wonder if this can be fixed with a cron job checking for connectivity or something else?).
The XBMC interface runs pretty quickly, though between screens it takes a while (likely due to swap use, though I've not looked at what it actually does to the system during running yet).
Network browsing again is slow (i.e. listing large video plugin lists), but acceptable, and adding a UPNP source works just fine. Doesn't seem to decode from Windows Media Centre; going to look into this later. I'm pretty much decided that this will be my media centre of choice though - once I've figured out a way of getting BluRay and surround sound in my lounge without the PC. Hrm... :)
Kudos to OpenElec for getting such a useable and solid system up and running so quickly after release; let's hope it goes from strength to strength.