2 and 3 are regarded as the better of the original series. If you're interested, definitely pick them up. After that the series kind of breaks down and fans opinions really start to differ. I like 4 and 5 a bunch, but I know plenty of people who say that's where Megaman started to become stale.
@believer258: lol I remember when Might & Magic X was first announced, people were saying "it's copying Megaman!". I was just wondering that being RPG gamers, you'd think they'd know their roman numerals.
Difficulty-wise how those megaman X compare to Super Meat Boy?
Megaman X has a nice difficulty curve that eases you into the game. I don't think it's really comparable to Super Meat Boy, but I'll say it's easier.