At the time it came out I wasn't sure what I thought of card-style games, I'd never been interested in Magic the Gathering that much (or Pokemon cards, Yugioh, whatever), so that was one reason that I held off for a while... (Though I actually do like several GC card-battle RPGs -- Phantasy Star Online Ep. III and Lost Kingdoms were both fun...)
Anyway, I did get BK later on hoping that it'd be good, but didn't like it. The game has what are probably some of the most unlikeable main characters of any RPG ever and some of the worst voice acting ever sure doesn't help any, that's for sure... I got the first one hoping that it'd be good, even though I usually don't have a strong interest in card-battle RPGs, but yeah, I didn't like it at all. The graphics are nice and it has some original ideas, but I just hated the characters so much, and the battles weren't very fun either... I was pretty glad that Tales of Symphonia was the one I got for full price and BK the one I got cheap, because I didn't like it very much. Like I often do with JRPGs I got bored with the gameplay (I'm not a huge fan of the genre, but there are some that I have liked), and there was nothing there, like interesting characters, or even just characters that weren't annoying idiots doing stupid things, to make me want to keep playing.
As for the battle system, it was a little interesting, but the whole "lining up numbers on the cards" thing was way too confusing for its own good, I thought. Evidently the second game gets rid of that. Good riddance. Also, stuff like how all your items go onto cards, items can spoil, etc, were interesting features, I guess. Added some frustration at times though I think. I don't remember, I haven't tried playing the game in a long time... I just lost interest partway through, stopped playing, and never had much interest in going back to it.
Overall despite how much I disliked it, trying to look at it impartially I could see giving BK a decent score because of the interesting elements of the game, but it certainly has some big problems, particularly in the plot and character departments, and I just didn't find it fun to play.
I don't know if I want to try it again, I lost most of my GC savefiles last year so I'd have to start over... that does not sound fun.
Anyway, my dislike/lack of interest in the first game did not exactly make me want to get BKO, so I never did. I'd guess that beyond the game's other problems, like coming out near the end of the GC's life, the fact that while the first game had sold decently a lot of people had had real problems with it also hurt its sales, I'm sure. Why buy a sequel to a game you didn't like all that much? Even so, given how much praise BKO gets I might get it sometime,
but I just don't want to spend like $30 for it... if it was cheap I'd consider it, though. From what I've heard it does improve on some of the things I disliked most about the first one, so I'd probably like it more than I did the first game, at least.