Damn, I love this game.
I find myself playing co-op most of the time as the games aren't too intense and are usually full of great people willing to help you out, however when I'm in a crazy killing mood I might have the odd game on quick match.
Is there single player? Of course, silly. This also includes three different modes: campaign, training and tutorial. These don't get you multiplayer XP, however, they allow you to gain items and happy stars (only on the campaign) and practice your skills. I go on training a lot to get myself in the mood and feel good about myself when I place top at the end of the game!
The campaign levels can be unlocked by leveling up on multiplayer.
Personally, I really like how the Happy stars and happy tickets are the currency of the game. Happy tickets are purchased by microsoft points to buy items in the shop, which I think is a bit bizarre because nobody can buy good items without spending real money, but that doesn't mean you can't get anything. Happy stars are gained in games, completing the campaign and selling unwanted items. They can be spent on the happy spinner (which I think should have better items), leveling up items and editing or removing item buffs, which brings me to my next point.
The item buff removal and editor is fantastic. I love it. It allows you to customize your weapon and make it as great as it can be. It's so easy to use and without it I think the game wouldn't be as good as it is. Adding a buff requires an empty slot which is either already there or can be made by removing a buff from the weapon. Some buffs are positive and some are negative, the level of the buff determining its strength. The leveling up of items is in it's name, increase the level of your item to increase the strength and defense.
The game is always up to date, bringing in new events and items all the time and also includes a treasure map which if you visit you get a certain amount of happy stars OR if you've been visiting enough, a special premium item. It's so addictive I'm always on to get my daily prize and then kick some butt!
The character customization is limited (but you can choose what your voice is like!), but it doesn't really matter because you're covered in awesome armor unless you're wearing revealing armor. Hairstyles and hair colours are available for purchase in the shop for happy tickets, but there's still a small range of features to choose from which is fine for me.
Now for the downsides.. Unfortunately, there is quite a bit of lag and the matchmaking usually takes quite a while. I usually end up distracted while the game is matchmaking until it finally starts. I am patient, though, so I suppose it's okay for me. The game ends completely if there are too few players which I don't get when they can just be quickly replaced by AIs or maybe even allow other real people to join. This tends to happen especially on player match. It also kicks you out when you disconnect to players, which is understandable but it happens a bit too often.
Ah! I think I've covered about everything. If you haven't already played it, go for it. I think it's great fun and slightly annoying or expensive at times (a few of the reasons I rated it down a little) but it's quality for a free game. Thanks for taking the time to read my review!