I enjoy playing both AoC and the original HW by myself, it's a lot of mindless fun.
I've played the original in multiplayer a little bit and it was even more fun, as me and my friend could divide tasks (I could go on defense, while he went on the offense) to try to get gold rank on a lot of difficult stages. It was also the only way that I managed to get Agitha's Rank 3 weapon. That mission is impossible without a friend.
Saddly, I don't really hang out with this friend that much anymore, and I don't really know anyone else that wants to play that game with me...
But yea, they are a lot of fun in multiplayer!