Welcome Back, MegaMan!
Nostalgia aside, MM9 is beuatifully crafted in the classic megaman and 2d side scrolling style. Bosses with names that do more than hint at their powers are back in force! If you never liked Megaman, this is an easy bit of advice: stay away from MM9, it's as true to the series as an original NES game.
One area explored in MM9 for the better is a backstory with a bit of a laugh in it, and more modern feel. Lets face it, we used to be fighting to quell robot uprisings... in MM9 we're doing it essentially as a Public Relations tactic. Of course, if you're expecting 3D cinematics with 8bit gameplay... think again. This is old-school from the start to the finish, and while I love that, some need to know up front that for them, old-school may be too old.
The other caveat is that MM9 is every bit as punishingly hard as any other MM game on the NES was. I relish the challenge, but it is very much out of step with modern "forgive-me" gaming, and may piss some off. Be warned, or be enticed, depending on your taste.