Just played it yesterday, after having the game for a week. Loving it already, my team is generally some mix between Ryu/Frank West and Ippatsuman/Viewtiful Joe.
Frank's Mega Buster really comes out quick... so far I haven't seen a projectile super that comes out faster, besides maybe Megaman's; it's really good to punish attacks, which is good because you can usually hit people out of anything with anything in this game. Viewtiful Joe's air combos are immensely satisfying, as are the way you can link Ippatsuman's ground moves.
Yami's... interesting. It's sort of like fighting Abyss but with grabs and you can't jump over him. Harder, but it only took me two tries to finish him the first time. Hammering him, making sure you have at least one character with a projectile super, and blocking when he pulls out something big (unless it's a grab) seems to do the job. Again, a lot like Ryu.
Alex's Hyper Bomb is just dope (it even grabs the giants, that's just friggin tight. It's a shame he can't do it to Yami), though one of my friends is an Alex player and it's extremely quick start up is a very irksome threat, as is his high damage output and powerful attacks. Luckily he's very easy to punish if you're good at spotting.