This one is pretty fun!
I was actually avoiding it because of some bad reviews of different sites that I trust; but alas, there are just so many characters that I love in it, like Medaka, Nube, Seiya, Joseph and Jonathan Joestar, etc. So I decided to get it, and here is what I think about it:
It's not a deep fighting game, the mechanics are quite simple; however, there are some assists and character combinations that can be exploited, for example, Kenshiro is a deadly assist and I've seen a lot of Madaras using Katon like there's no tomorrow, oh, and many Aizens as well. However, those are usually found in ranked matches and 'fake' player matches (fake as in not playing for fun, but for competitive reasons).
I was worried about the story mode because a reviewer said that the dialogues were terrible and it felt like it was a single person doing all the dialogues; however, although it's not the best in terms of writing, each character's personality does show. If you liked the interactions in Project X Zone, this is a bit below that level but still enjoyable.
Online player matches are just great, everyone is going nuts choosing different character combinations, and there's always someone ready to play; for me, it's taken like half an hour to find a ranked match, and a lot of players in rank disconnect when they are about to lose, so, I've started avoiding ranked alltogether. So, Player Matches are awesome right now.
I've just played the game for 10 hours or so (not non-stop, but it is addictive!), and there are many things I have yet to see and learn. So, I highly, highly recommend that you at least rent this game, I haven't regretted my purchase
Edit: Uh, I just noticed that there was another thread about this game. Oh well