Quite fun, although it may be a bit repetitive. I did the entire campaign with a friend through Hamachi, but it was quite... buggy. We've had disconnections/desyncs/other problems over the course of its entirety, and in totally maybe about a hundred times. HOWEVER, it was still a relatively enjoyable experience--I've had a ton of laughs with what we sometimes do.
Although... I really do wish that there was some sort of diversification with what role each player does--or rather, with what unique capabilities that each player had... which was, of course, nothing apart from staffs and swords (which honestly weren't really significant, and dying off-screen meant you lost those anyway).
My favourite spells were obviously the popular QFAAS, both as a beam or area-cast, QQQWE (I think, and thanks Foogs) for crazy water mines that propel anything that triggers them to outer orbit--this was mostly the source of my laughter, and our unique use of QRAASE (I think again) for ice rock things with lightning that would strike anything nearby and explode shortly afterward. QFAAAA or sommat would also be in relative use, as these were long-term steam clouds with lightning. Both do upwards of a thousand damage, matching the power of QFAAS, and could be used as a weapon-cast.
One of the most hilarious things I've seen and done was using water mines on Vlad who proceeded to fall over and out of the level during his boss fight.
Arena... that's a tricky one. I don't think there's enough content here for replayability, as there are only 20 waves on either of the levels, and they've already used up most of the normal monsters during these fights. Of course, they could create a boss run, but meh.
Overall I was impressed with the game, being a $10 pricepoint and of course, being an indie game.