Really, the worst thing about Metal Slug 2 is the price. At $3.99, it's more expensive than most games on the App Store, but if you have the cash to burn, we feel this title would make a fantastic edition to your iOS library. That being said, jump into the warzone and come out firing.
By continually emulating, instead of innovating the METAL SLUG series, SNK PLAYMORE and DotEmu feel like their hands are tied when it comes to replicating the performance and precision of the Arcade originals; METAL SLUG 2 may not be the strongest of the originals, but it plays better on the iOS platform than previous releases and thus wins out by default.
Metal Slug 2 is a fun experience, enhanced further by bluetooth co-op multiplayer and a Mission Mode, which allows you to practice stages which are giving you a difficult time on their own. The only real downside to this version is having to overcome the various hurdles the controls sometimes present, but that's still a pretty considerable hurdle.