Well I just got it yesterday, The singleplayer is fun for me but probably because it's my first time through. I generally hate single player games so I'll leave it at that.
It's incredibly easy to pick up. I've only played a total of maybe 5 hours and I had the entire game down after maybe 10-20 minutes of play. I jumped right into a match against my brother and it was easy for both of us; neither had read the instructions or done the training mode. Even after reading the instructions and doing the training there were maybe 2 or 3 little things I didn't pick up on my own playing against my bro.
I think it would be a great party game. The most players I've played with so far is 2, just me and my bro, but we both had a ton of fun. I'm going to be bringing it to my friend's place for our (incredibly nerdy) new years party. To me the game is reminiscent of Super Smash Bros. style and quality fun (except it's not a fighting game), but this might be because it is new to me.
When I first started playing online and had 0 points I was ranked something like 13,XXXth, so the online following isn't gigantic but it's definitely decent. Usually takes about 30-60 seconds to find a ranked match, it's finding one with a good connection that takes some time.