I'm pretty sure the online in Slug 3 is about the same as most of SNK's other games. Not very good.. The Konami beat em' ups are all great. Very simple, but if you got some buddies to play them with their well worth it. X-Men and The Simpsons include Japanese ROM sets so it's almost like two games in one. The online in all three Konami games depend on the host's connection. If you've got good internet and your close by other players it'll run alright, but I've played in a lot of VERY laggy games and people leave often.
Scott Pilgrim is pretty good not having online killed it a little for me. It's a great game with buddies but other then that it's like a more simple River City Ransom. Castle Crashers is also very good, but I played the game to death and I'm sick of it. The problem with that is every character fights the same, they just have different magic attacks. Most of them are the same, just with different effects. You can spend a lot of time trying to collect all the different weapons and animals. For me, I just played it WAY too much.
Also, check out Guardian Heroes if you're on the 360. It's well worth it and the online is pretty good from what I've played of it.