Magicka was pretty unstable early on, but seemed to be a lot more stable when my friends and I last played last year sometime. I don't remember any crashes.
It's perfectly winnable alone, though it plays a bit differently since the enemy's are always chasing you specifically. Can get a bit frustrating with certain enemy types, but there are protective spells and walls which can fix it.
If there's a demo I'd recommend picking that up. It's a really neat concept which works much better than I would have expected. Kinda like a fighting game, you can get through by spamming certain spells (earth earth earth earth pewwwww) or you can do some neat stuff (earth earth earth earth shield). Unlike a fighting game, every combination of spellbits does something. Sometimes it sucks, sometimes it's great.
I tended to use water to knock enemies back, then freeze them. Helps a lot in single player. Didn't go so well in co-op since my friend loved fire magic
But like, you can add some "arcane" to those strategies to make them energy beams, or "earth" to infuse them into projectile rocks. It's pretty neat.
Edit: Oh yeah in multiplayer you can "cross the streams" of beam spells. We never got used to doing it on purpose, and sometimes it went horribly for us, but it looked cool