The classic arcade shoot-em-up.
Keep note that this version is a port of the iOS version, so while it's not exactly a perfect arcade port, it still plays and handles rather well. There are some slight sound anomolys with the emulation, but quite frankly it's as close as you're gonna get with the true METAL SLUG experience on Steam.
As for the game itself, 3 is known to be the best (and the hardest) of the series. And after 14 years, this still holds true. Saying that, once you learn the patterns of the enemies and bosses, and know the best routes to take and where the weapons and vehicles drop, you'll be mastering the game soon enough.
Online play seems to be pretty solid, provided you and your partner have good connections to each other.
Overall, I would say; Yes, buy this game if you're a fan of the series. If you're looking for a better emulation version, though, see if you can pick up the Metal Slug Anthology for the PS2 or Wii, which has the games 1-6 and is a really stable port.
This version is still perfectly playable despite the slight emulation annoyances.