Amlogic is known around the XBMC/Kodi forums for their excellent OSS VPU drivers and boxes using this chipset are usually among the most recommended.

This new chipset (S805) should also be able to support H.265 videos, the first patches for it have already appeared on the mailing list.

I can't comment on VDPAU support but for Kodi this board should be pretty awesome :)

This should run most of the existing Ubuntu and Android images without any hassle. Having owned an ODROID-U2 for a long while now, I can tell you that these things run Ubuntu 14.04 like a dream (except for the lack of OpenGL - you only get ES) and Android works great:

http://the.taoofmac.com/space/blog/2013/02/10/1230

(Mine is now running Ubuntu only, and the only gripe I have is that the kernel currently lacks enough group support to run Docker - that's only a recompile away, but I like my uptime...)

I have the Odroid XU3 (which I think runs the same version of their Ubuntu). XBMC/Kodi runs on it, but I get a crash intermittently when fast forwarding a movie. VLC works perfectly. Chromium and Firefox runs superbly. I was unable to get Flash working so Amazon Instant Video and HBOGo don't work. Wireless with the $8 usb dongle they sell works perfectly.

Have you tried the pepper flash player?

http://forum.odroid.com/viewtopic.php?f=8&t=2374

You should check the ODROID forum for newer graphics drivers. The bleeding edge versions are released every now and then as separate binaries/images.

A lot of people in the HTPC community are buying these things for just that purpose. This board should be excellent as a HTPC.