I HIGHLY recommend this game, especially if you have a friend (or two, or three) to play it with.
There are two game modes - Adventure (story mode), and Challenge (arena). Both are fairly simply in nature, but they provide some decent challenges and a fairly good variety of enemies to fight against. People that rush through Adventure mode, skipping all of the dialog and not exploring for secret areas / weapons will probably beat it and put it down after a few days.
One of the best aspects about this game is the spell system. You have five slots to choose from 8 main elements (and two "hidden" elements), which will create a spell. Some elements benefit more from each other, while others cancel each other out. While just plain fire will only make a fire "stream" out of your staff, fire and earth together can make a fireball, for example. Once you have combo of elements selected, you can either right click to cast it normally, shift + left click to apply it to your weapon, or shift + right click to do an AoE effect. There are literally thousands of possible combinations, and several dozen of those are extremely useful. Additionally, you also learn special, extremely powerful spells from books (another reason to explore), which require a specific combo and hitting the space bar to cast them. What makes this so fun is the fact that you really don't have time to think about what you're choosing. With enemies quickly closing in, you just need to start nuking things and sorting it out later. If you're really good with memory, you can throw together some great combos. Button mashing doesn't work well here.
Single player is decent fun, but multi-player is where this game REALLY shines. The fact that you can just as easy blow up your friends or yourself with a spell means that one person is fairly easy to control, but when you have two, three, or four running around it turns into great chaotic fun. My brother and I played it all the way through, and there's nothing quite like unleashing your ridiciously overpowered spell on a horde of monsters, only to have it blow up your party instead. It may sound lame, but trust me, it's a riot.
I'd give the game a 8/10, right up there with what I'd give Amnesia, Mount & Blade, Trine, and Plants Vs. Zombies. It could use a little more length and some extra weapons or variety in some areas, but it definitely is fun, funny, and overall a great time.
For $5, this is a no-brainer purchase.