It's hard to rate games like this since the game itself isn't the problem - the port, however, is. Metal Slug is an old arcade favorite and is such a great game that it would likely still be fun played on an Etch-a-Sketch. Unfortunately, this PS2 anthology is barely usable.
The games themselves are fantastic, chaotic fun. Playable by two people at once with fantastic animation and hilarious settings, they're terribly compelling.
However: Why would a 2D game designed for arcade machines have to load for 5-10 seconds on every level switch? Why in Metal Slug X does the game have to pause and load every time you select a character from the main screen? Why does pausing the game and then unpausing it cause the music to loop indefinitely and then the game to crash? Why is there a slight but somewhat noticeable delay with every button press?
As I said, it's hard: The games themselves are, of course, excellent. The port, however, is shoddy and obviously done as an afterthought.
There isn't much that I can say about this collection. It is packed with 7 different titles from the MS series. And while that may seem like good bang for your bucks, the experience hardly has anything new to offer, as you transition between each title. With no multiplayer mode, or other arcade-like modes (besides playing alone or with one other person in the story mode), you might end up playing through this only once. Unless you are a hardcore fan of the series, you can pretty much skip this one for good (or rent it at the very most).
If you like Metal Slug it rocks. if you do not, then its pretty lame. I rented it without really paying attention at the title, I was bored after about 5 minutes. Only rent it if you liked Metal Slug in the arcade and want to play it at home.