I really liked this game, actually! Slizer is a surprisingly fun and well-thought-out addition to the playable roster (Certainly more balanced than Mega Man & Bass in that regard =P ), and I loved the weapon puzzles for upgrades sprinkled into the stages. It's probably the most fulfilling version of any upgrade system I've seen in any game so far.
I will say that when I started playing, it wasn't really intuitive that I could switch between Mega Man and Slizer in the quick-switch menu, and only in the quick-switch menu. I wound up really liking said menu, it was a lot easier to use than the weapon wheel in Mega Man 11, but the fact that there was no option to switch characters in the pause menu proper confused me.
Also, I found a bug: the upward screen-scroll transitions appear to only work when you're in a ladder-climbing state, but since you can jump off of ladders in this game, it's very easy to jump up past the spot where the transition would trigger and be stuck a screen above without the screen having scrolled with you.