My experience; pros and cons:

Raspbmc:
+ Used latest full image file (not the online installer version) and was up and running in minutes.
+ Wireless adapter worked (after an hour or so!) without any extra drivers – until it cooked itself and died after a couple of weeks (shouldn’t buy cheap unbranded adapters!) – I’m now properly wired!
+ I could enter MPEG2 / VC2 license directly via the Raspbmc setup program
+ Playback quality (max 720) has been flawless
+ I have Youtube and iPlayer working well, if somewhat slowly!
+/- I can access my HDD on the pi via windows explorer to transfer files back and forth but it took me days to get it sorted and I’m not entirely sure what i finally did!!!
- menu response is slow, sometimes painfully so.
- I miss the first 5-10 seconds of video for most avi/mov/mp4 files – the sound starts ok but the time it takes to clear the menu and start playing the video is annoying. I have to use the left cursor to skip back to the beginning if it’s crucial.

OpenElec:
+ OE Menu response was initially a lot faster than Raspbmc but noticeably slowed down a bit after I had populated the video and music libraries.
- I could not get the wireless to work (I had Linux drivers but I couldn’t figure out how to install them from a windows machine)
- To play MPEG2 (DVDs) you need the license but you can’t enter it directly on the RP via the menu system (unlike Raspbmc). I put the SD card into a PC to edit the ‘config.txt’ file and opened it in notepad (default prog) where it looks a complete mess (due to CR and LF issues). After a failed attempt I opened it again using good old Wordpad which showed it formatted correctly and I realised I had to remove the comment characters from the beginning of each line for the codes to work!

General comments:
+ The movie database works really well (if slowly) – I have found the info and cross-referencing (stars) to be very useful.
- Both systems were quite slow at populating the library database (at least 10 or 15 seconds per film) – seems to be an XBMC poor programming issue? I have disabled fanart to improve speed (in skin settings)
- Both systems were slow at presenting search results on Youtube or iplayer – again, an XBMC poor programming issue?
- TV programmes released on DVD are a problem. I think they expect you to rip each episode but I just rip the complete DVD and cataloguing them doesn’t work well as they are neither movies nor TV episodes.
- The music library is rubbish compared to most PC media players (I use Winamp). It catalogues lots of ‘various’ CDs with each track as a separate album.
- No functioning fast forward or rewind is unforgivable these days! The skip forward/back controls work but really… My old Danelec box worked flawlessly at this.
- DVD menuing is flaky to say the least. Some don’t work at all, some partially work whilst others are fine! My old Danelec box worked OK with all.

I use an ultra mini keyboard/trackpad/laser-pointer to control the box – £20 but it’s absolutely brilliant and the size of a normal remote control.

Only 1 hour on openelec and everything worked great. That was my experience.

Wish I had read this page first. My raspbmc SD cards will now be reformatted to openelec.

OpenELEC is installed on a 256MB pi @ 720. IMO it performs better than Raspbmc and just maybe Xbian with this amount of memory.

Xbian is my favourite and runs on my third 512MB pi @1080. To me it is the smoothest and most refined but there really isn’t that much between them. I would be happy to use any of them and would recommend all three of them.

First of all sorry for my English. This is the best article I have found to see the differences between this three distros.
I am now using raspbmc. It’s quite good but I am going to begin to use my raspberry Pi for other porpouses and I prefer to be sure that I can do “apt-get update/upgrade” whithout any problem. As far as I can see, the only distro that let me do this is Xbian. Am I right? Thanks.