And note: mpeg1/2 don't need any acceleration. Even the weakest crappiest cellphone CPU can handle those just fine. Even most h263 is fine. The only thing you REALLY need acceleration for is h264 (AVC).
XVBA, just work with a few video cocdecs (backend under VAAPI), just with with HD4000, and the programmer has explicity said on this forums that XBVA driver is plenty of bugs. Maybe if some day ati implements it correctly it will be fully functional. The hardware has the potential is just the api & driver is not yet working. And it seems that it will not work in short term
VAAPI I don't know exactly how good bad it is, but i guess is worst than VDPAU.
I am not speaking about the APIs itselfs, but about the API and Driver implementation and functionality together.
VAAPI has the advantage of being a UNIFIED API that stands in front of ALL the other existing implementations. VDPAU as an API is a FAILURE. VDPAU as a backend, in MY experience, is also a failure. That's two failures right there. Again, mileage may vary with luck, but it would be inaccurate to present it as something that *just works*, because it DOESN'T always work.