You can never have too much of old-school arcade action!
Now you have 6 characters to choose from, each with a slightly different perk. The weapons are more of the same, with some new ones I think, like the tesla shock weapon. Some new machines include a lame hopping ostrich and a mech, with one of the best boss fights in this game via mech vs mech. Talking about boss fights, Metal Slug XX features really creatively-designed bosses that are equally ridiculously hard to beat.
With 7 missions in total, this newest Metal Slug is an unfair and lengthy arcade adventure which will guarantee to spend a schoolkid's whole week of pocket money to complete, though in the PSP, you'll find that the fun ends in unlimited continues and a mere hour or two. I haven't tried completing any objectives in the Combat School yet, so maybe that'll keep you occupied longer for the hardcore fans.
KingLim