Very.
I got many PSX games, even more PS2 games, some Gamecube games, very many Wii games, many PS3 games. If PS4 pulls some crap again, I won't get it (or reeeealy late). I also accept additional BC component for sale, so that they won't have to increase the regular price, but I will only accept proper 100% hardware backward compatibility - nothing else.
And Wii/PS3 digital games should be transferable onto Wii U / PS4, otherwise no deal. Although I already prepared for that case, I didn't buy that many digital games and always prefer discs.