Dan is right on the money in saying that the lack of extras in Metal Slug 4 & 5 is very disappointing. This is my third review of a Metal Slug title for GameCritics, and over the last few re-releases, the decline in value for each game has been fairly obvious.
Metal Slug X for the original PlayStation was the best effort so far. Not only was it the same hard-core action and beautiful animation that the series is known for, there were a slew of skill-testing mini games and art galleries to unlock. Clearly, the developers went above and beyond when they put this project together.
Metal Slug 3 wasn't as fully-loaded as X was, but it did have a few perks and because the developers eliminated the unlimited continue system, it gave an extremely serious challenge. After sweating and working my fingers to the bone, I'd have to say that finishing that game was an achievement I'm proud of— I look back on 3 with fond memories.
Metal Slug 4 & 5 are great because it's always nice to get two games with one purchase, but compared to what came before, SNK Playmore definitely took the no-frills approach. So, while I did enjoy my time shooting zombie vomit across the screen and piloting one wacky vehicle after another, I echo the sentiment that there should have been more.
On the plus side (at least, some will see it as a plus) the infinite continues are back, so people who don't feel like going insane can actually make it through these games. It's always nice to have a friend along as well, and old-school two-player gigs are few and far between. For this feature alone, it gets props.
Besides giving it an extra point, I agree with Dan's opinion all the way down the line. It might make for a boring second opinion, but Metal Slug is Metal Slug—you either love it or don't, and I do. Although the world of videogaming presses ever onwards (and SNK is being left behind), it's nice to see that some things are just as enjoyable now as they were then.