I don't have any experience with the UG802 (Rockchip 3066) but from what I've read the Amlogic 8726 based devices might be more useful since they support hardware acceleration for XBMC. The other Chinese ARM processor manufacturers such as Allwinner, Rockchip, Telechips etc. have so far not been willing to release the necessary software to enable graphics acceleration. Personally I would rather choose a larger device since these really small units tend to get very hot and often crash as a result.
Been playing around with one for a few weeks. There are a lot of info and reviews on this device and basically works as advertised. It does have a few issues, though. The two key ones are wireless connectivity and audio-video sync.
There are ways to overcome the weak wireless signal. I'm using my old Telkom router as an AP (which is physically close to this device). The audio-video sync issue is being worked on by a few developers (google 'Finless ROM' and '2dark4u ROM').
I streamed 720p videos from my laptop via wifi, with no lag. Also no problem with 1080p videos on a micro sd card, but it seems like it might have output resolution fixed at 720p.