Please, do your PSP a favor and buy this game.
Ok here is a little history of the series: it was first released in the arcades on Neo Geo MVS systems. Remember those arcade setups with 2 or 4 different games on them mostly fighting and shooting (Schmups à la Raiden) usually? Well these cabinets also had an home console counterpart. Metal Slug 1 was first released in 1996. That is AFTER the Playstation was released, so don't let those old school 2d graphics fool you. In fact, the Playstation could not even run the first Metal Slug properly. It had mid level loading, limited animations and the usual slowdowns when too much stuff was blowing up. The first decent versions of any Metal Slug were only possible on PS2 and XBOX, machines that came out more than 10 years after the Neo Geo! How's that for old school graphics?
So the skinny on the PSP version: It's not perfect. It cannot be. The PSP does not have enough RAM and the reading speed of the discs is too slow for the game to be flawless. However that does not mean it's not good. Actually, It's a great port and probably one of the best games you can buy for this handheld. The good? Well most importantly, the animations are arcade perfect. That means they did not skip a single frame to lessen the burden on memory. The animations are perfect and my are they beautiful-- especially on the PSP screen, everything comes out very sharp and clean. The best part I think is that they managed to cram entire levels in one loading time, with some exceptions. Hidden passages, for example, require some loading time. The bad? Well it's really just the loading. It's not very long, but it's there. It's nothing compared to some other PSP titles but the frantic action of Metal Slug sort of makes you damn the loading screens for the interruption of fun they cause. I had issues with the PSP's D-pad but you can also use the analog stick so whenever more precision is required, I simply use it instead.
The best part? You have the entire series for 40$. That's like a gift from the heavens. 7 full games, for less than 10$ each. You simply can't beat that. It's very generous of them to have put the entire series on one disk and that's why my headline urges you to buy the game. SNK Playmore deserves every single penny they charge for this compilation and it will likely be a game that you play for years to come. The value is nearly second to none.