Well, a bunch of that is a matter of opinion. Crap legends? Some people like them, some people dislike, simple as that. Same as your claim of uninspired music - while some of the legends did look pretty similar to each other, I really have to disagree that the music was 'uninspired'. Same goes for designs for Pokemon and protagonists (which is a pretty darn minor reason to not get a game, let's be honest). But each to their own.
Next to no post-game activity
This is more objective however. True for BW, but certainly not for B2W2. (Musicals, Movies, Join Avenue, Battle Subway, medals, white hollow/black tower, dream world, missions... I think it had a
lot to do.).
My advice? If you find it cheap, go for it. You'll add a bunch of Pokémon to transfer over to 6th gen games when Bank finally comes out, at the very least. And there were a bunch of features in the game I wish were in X&Y (medals, join avenue, even many parts of the dex. Being able to see what you are missing for each
part of each route? Dang). My preference is B2W2 due to more content, and it's not necessary to have played BW to be able to play its sequels. (It does help, but you won't get too lost on the plot, I'd wager).