JSS is an awesome game, you guys are crazy :P
It has a fairly complex unlocking system (actually, there's just a learning curve if you can't read japanese), and the adventure mode can get tiresome after a bit. But multiplayer is BLAST. It's a shame it isn't online, but the new one will be, so you may just want to wait for that.
Adventure mode is basically a single player mode where you travel through the Jump Universe and fight battles / complete missions on the way. Many are quite easy, but some can bet really difficult. Most missions have several different tasks you're asked to complete, but you only have to complete the first one to continue on. But the more you complete, the more you unlock.
Check the videos
here for a little more info if you haven't seen them already.
$49 used is TOO MUCH.
You can get it new for less than that from Play Asia and have it shipped for like $6 or less and have it in a week (give or take a few days).
Personally, I love the game. It may not be much as a standard fighter, but if you like Super Smash Bros, it's the closest you'll get on the DS for now. I like it better, to be honest. Just make sure you study the FAQs really well or else you'll get nowhere.
I highly recommend getting it, even at the $50 price tag (if it's new). And by the way, if you have a Fry's near you, the ones down here have JSS, Bleach DS, Naruto 4, and lots of other imports in stock now as well for the usual $49,95 price. So look there first if you have nearby.
Jump Ultimate Stars (the next one) is coming on Thanksgiving, if I remember correctly. If you're still undecided, wait and get that one first. It's sure to be a hell of a lot of fun.