I was disappointed with Castle Crashers. That's not to say it was bad, but it was so overhyped that it's hard to not be affected by that. I tried my best to put that aside and found the game enjoyable. Sure, it's ridiculously repetitive and some of the humor is juvenile. At the same time, it's a great example of a traditional beat 'em up with enough smaller additions that replaying it makes sense.
I find it hard to play for long because at times it just seems to go on and on and on. Some enemies take way too many hits, largely because I think the game relies TOO heavily upon its leveling system. Some people love getting multiple characters maxed out, but I just find that tedious.
I personally never had any issues playing this online, but obviously many have.