I like it a lot, and I'm not huge into the 2D castlevania games (I used to love them on the GBA but they were the same thing over and over so I stopped). I really like this one though because of the WHOLE castle being on screen at once, with all the monsters active, and seeing your 5 friends all wrecking shit at the same time. It almost reminds me of Todd's Adventures in Slime World, an amazing co-op side-scroller that nobody played because it was on the Lynx.
That being said, I'm not too sure of the longevity of it. I was getting into some games where they were just breezing through the levels in about 6 minutes or something. That might have been because it was set to normal difficulty though.