Backward compatibility works as well as advertised, Super Meat Boy, Defense Grid, Geometry Wars and Zuma all appeared in my 'ready to install games' area and all play well. Games with Gold titles transferring too, friend said Viva Pinata appeared, that was the only reason he owned it digitally.
Generally agree with the 'often wanted but seldom used' desire of backward compatibility. I did that with my PS3 that's still sitting in the bedroom and the 5-6 PS2 games sitting in a drawer I never got around to playing on it. I have no desire to play last gen games, but when they're available digitally and fairly timeless due to art direction or fun factor (Super Meat Boy and Geometry Wars in particular) then they'll get played periodically for sure. Zuma I'll probably uninstall, just picked up the copycat that released last week.
My friend is still #1 in Geometry Wars, the bastard.