I have both a 3600 & a 3610 Revo and have had Ubuntu full & minimal, XBMC standalone, XBMC-Live, XBMCFreak-Live, Windows XP, Vista & 7 on them at various times.
(I'm an inveterate tinkerer, in case you hadn't guessed)
The original 3600 (Single core Atom 230) was pretty damn good with Ubuntu/XBMC in whatever combination.
The XBMCFreak iso installed to HD from a thumb drive was by far the easiest setup - it was complete in about 10 minutes and pretty much everything Just Worked[tm] right away, including all the various niggles seen again and again in these forums - MCE IR Remote Receiver & Logitech Harmony, analogue & digital sound via HDMI, full 1080p playback, connection to Windows SMB shares etc etc.
Windows on the 3600 was ... just about `OK`. Useable, but nothing you'd want to live with for extended periods.
My family was deeply impressed when they saw XBMC in action when visiting one day - so much so my sister asked me to make her one too. I gave her my 3600, and took the opportunity to go buy a 3610 (identical, but for dual core Atom 330 & an optical SPDIF port), plus a cheapo 30Gb SSD.
This coincided in time with Dharma getting close to release, so I've been fiddling with various beta & SVN XBMC builds on an Ubuntu base. (Didn't bother installing a stable Camelot release and upgrading in place)
It has meant a bit of tinkering round the edges to get everything working as smoothly as before, but now everything's fine and more and more of these tweaks are becoming extinct as the issues get fixed and built in.
I fully expect a fresh install of Dharma/Live when it reaches public release to be equally as pain free on a 3610 as the old 3600 was with Camelot.
Windows on the 3610 is noticeably improved - that 2nd core really helps the whole experience feel more `snappy` as the background system processes don't bog down the desktop UI response. It's still not going to break any speed records data crunching compared to a beefy desktop machine and playing games is a decidedly second rate experience, but then again that's not really it's design purpose.
The 3610 + Ubuntu/XBMC Dharma-Live + SSD disk is really the whole home theatre PC experience ... with a cherry on top
Everything is blazing fast and lag-free and completely responsive, even on heavy load skins like Aeon, Transparency etc stuffed full of extra artwork and menus.
That said ... it's a perfectionist's setup. The 3600 really was completely fine for me for 99% of XBMC tasks.
(especially so with the <useddsfanart> advanced option that's now in recent builds, including Dharma. This is a godsend for puny Atom/ION boxes as it converts standard compressed images to a `texture` format that takes *much* less CPU effort to decode and render. Gone are the days of display stuttering and skipping images and generally lagging as you scroll through big lists of backdrops/fanart/posters/covers etc etc)
So ... to sum up what has turned into a surprisingly lengthy post ...
A Revo 3600 with the XBMCFreak ION tweaked edition of the Ubuntu based XBMC Live CD is easy peasy and working fine right now.
If you can wait a couple of weeks I'm pretty sure the 3610 Dharma version of the same install will be in the same state.
Keep Windows on the machine as a dual boot if you wish, but I only used it as such about 1% of the time. The vast majority of the time was spent as an Ubuntu/XBMC set-top box `appliance`.