Yep, I'm very much enjoying the game as well, though I'm not sure if I personally rate it as highly as you do. There are some minor issues I have with it that keep it from being an all-time classic for me, including the fact that I can't play that game for long stretches.
I think it might be due to lack of variety. Whereas in X-COM you have base building, research and whatnot to break up the combat, there's really none of that to be found in Mario + Rabbids. It's almost purely combat, with some light puzzle elements to break things up once in a while, and that can take its toll, especially with how long some of the combat sequences can take due to the inflated HP pools that you and your enemies have. Granted, it's the only way to make the game work, imho; due to how mobile you and your enemies are and that you can damage each other with dash moves, you'd be constantly dying if your HP pool was a lot lower.
I still think it's a ton of fun, but I find myself consistently having to put it down and do something else after playing two or three maps, whereas I marathoned the hell out of X-COM Enemy Unknown when that came out.