Can't speak to the Online version, but I bought Black a couple months ago. I've also got Twisted Metal 2 which was excellent. I love Black as well. If you like to blow up animated vehicles with some really sick storylines, you're going to love it.
I don't think the Twisted Metal series will ever recover and make it back to the heights of part 2 (one of the best games for the Playstation), but Black is at least way better than parts 3 and 4. They were big missteps.
On its own terms, Black is pretty good, though. The online version plays a bit like an old Quake deathmatch most of the time, but I couldn't quite figure out how to set up a room for one of the other game types and keep it open right (so people could join and start a larger game) when I had it. I haven't actually played it in a bit, I'll have to give it a shot again. Once you try it, I'd be interested to hear which modes you think are the most fun.
Haven't played Online...but plain old TM: Black is a good game. IMO, The pinnacle of the TM series is and may always be TM2, though. Just had everything going for it: Crazy Gameplay, varied and creative levels, and a sense of humor sorely missing in TM: Black.
Not to mention those kick ass comicbook-ish cut scenes! Fit the game like a glove...I really wish they'd go back to them.
When you say you sent off for it, was it from an ad somewhere in TM:B or the PS2 online kit or somewhere else entirely? I don't have the online kit, but I do have broadband and may get one when they come down price-wise.
For anyone that hasn't jumped on board yet. It makes sense. The game's been out for four years, so just about everyone who's going to buy the game has already bought the game. Time for a new audience. Give it a shot if you haven't already.