The Robot Master design was by far the most interesting to me (except possibly 2, but that one has loads of issues of its own), the difficulty is pretty tame, it has collectables, the last stage of "Proto Man's" castle was clever, and a lot of the enemies are more memorable than games like 1 and 4 (Yudon especially). Plus all the stages had cool backgrounds and things; much more so than the others except for maybe 6.
It's a good game, definately not deserving of all the bashing it gets (good, but not great... though still very fun). I think a lot of the bashing comes from the middle child syndrome (people hate 4-6 and give 4 & 6 various exemptions and channel all their back-half Mega Man dislike into MM5) or disgust with the middle (more hostility towards something with average scores because of how it stands next to MM1-4 and very demanding expectations. Not as good by 1 inch, they perceive it as a foot). It does experiment around a lot more than people give it credit for (find the letters, Beat, the jetski, swapping a water level for a space theme, the reverse gravity level, Super Arrow). Ultimately, all of them are fun (MM7 too) but different things in each game appeal to or repulse different people.