Street Fighter X Mega Man simply put is the Mega Man gameplay of the old (see: NES) fitted with Street Fighter characters as the new game bosses, new weapons, and of course, humor. What's fun here is the careful detail put into every level e.g. themed stages, great background music.
We're going to reduce the blabbering to a minimum. Here's the reveal trailer from Capcom itself.
Street Fighter X Mega Man Official Game Trailer
Street Fighter X Mega Man is free to download directly at the Capcom Unity website.
As fun and as exciting as it is, we're having difficulty downloading the game. It may be due to the influx of players on the website that's causing the problems, or some other (there was one point that my progress bar suddenly stopped dl'ing at around 50%). Giddy users are also reporting corrupted downloads. Well, whatever. File sharers are already mirroring Street Fighter X Mega Man, but make sure it's clean!
Happy 25th anniversaries Street Fighter and Mega Man!