After sitting down and playing the campaign through and through, this was a really fun experience! I liked the addition of the MM9 masters to the lab as you finished the game, neat touch. Found and fought Quint, and that was very difficult. It took me a minute to realize he was taking me to the future with MM10 and MMX stages, and the double Quint finale was pretty interesting. If there is anything I'd like to see, perhaps the Fake Man battle (after the chase) should be added to the memory scanner. Though I'll agree with points brought up by others, some of these bosses had some bad damage you deal to damage they deal ratios, and Quint's lack of checkpoints was just a tad infuriating. But, very well done none the less; the new weapons are outrageously fun to play with (Plug Ball is my favorite, it's like a faster Search Snake).
Final rating: 9/10. Perhaps the difficulty could be turned down for some bosses in v5B? At least give some checkpoints here and there.
Very fun nonetheless!