Rather than prepare a brand new entry for the PSP, Capcom remake the original Mega Man X with updated polygonal graphics and an even MORE rock-oriented soundtrack than before. (Actually, the PSP never got an original Mega Man game.) But it ended being more than a mere port. Aside from a separate difficulty mode where all the body upgrades were scattered into different places (and the enemies were more difficult to destroy, obviously), you could also unlock Vile as a playable character -- and his quest through the game, all the way to Sigma, is even more hellish due to his unusual weaponry system. Add to that a very neat half-hour anime called "The Day of Sigma", which highlights the events leading to the game's plot, and you have yourself a nice portable package for that X fan on the go.
Check out our Mega Man Maverick Hunter X review for a more in-depth look!