I guess it also depends on what service/site are you streaming,
Last time I tried free trial for netflix and hulu on my 4mbps, video quality on Hulu is better compared to netflix. Netflix can check your connection and adjust the quality of the video based on that, for me mostly I get SD quality or worse. Enabling the best quality on the netflix settings doesn't seem to improve much for me

Recently I've just upgraded to 8mbps and and now can stream HD trailers directly from Trailers 
I'm probably gonna give netflix a second try, with 10mbps I don't think you'll have any problems streaming videos much.

I rarely stream videos on the net actually, I mostly use xbmc on atv2 to play movies shared on my home network. On XBMC you can add a bunch of other video streaming plugins and not limited by what atv offers.