Not as good as 2, about as good as 3, better than 4 or 5. The graphics are 8-bit Mega Man and on the Wii they look crisp even on a 40" HDTV - I stopped registering the 8-bitness after the first 5 minutes. The music is pleasant but so far I've yet to encounter a tune that compares to 2 or 3. The bosses are not as lame as the later 8-bit games that introduced luminaries such as Mop Man and Lint Man.
This is a hard game and flirts with borderline cheap but never crosses the line. Every level has one or two hard parts with the rest being no big deal, and levels are very short once you can get through them without dying repeatedly. Being sent back to the start or midpoint isn't bad when it's a 30 second trek back and it cultivates a strong "I'll try just one more time" feeling. And you can always grind for items to make your life easier if you fail at Mega Man games.