One of the obvious challenges with Evolution is controlling your character using your touch screen. Of the three control options available -- fixed joystick, floating joystick, and classic directional pad -- I find the classic directional pad to be the most responsive, which is important when dozens of bullets are whizzing by. That said, whichever option you choose will likely take some getting used to. Fortunately though, there is an Auto-Fire option, which frees up your attention (and fingers) for more important things like running, jumping, and aiming.
For fans of the original game, the best thing about Evolution is that it stays faithful to its predecessor. Sure, graphics are updated, but the enemies and maps should all feel familiar. In fact, I even tried out some old-school tactics and they still worked like a charm.
My biggest peeve with Evolution is that it lacks a multiplayer mode. One thing I loved about the original game on NES was that it let you and a friend navigate levels and fire away at baddies together. You could back each other up when faced with enemies on both sides of the screen and even divvy up weapons upgrades to wield just the right combination of firepower. The two-player co-op mode from the past was like another game altogether, and unfortunately it's missing in this new iteration. Perhaps a future update will bring at least a Wi-Fi based multiplayer mode.
Lastly, I think Contra: Evolution could benefit from a tutorial or at least a page with some sort of gameplay explanation. As the game is now, some of the elements can be confusing, including the in-game currency system and the extra weapon slot on the bottom of the screen.
Still, with its fun, yet challenging gameplay that is, for the most part, faithful to the original Contra, Evolution shouldn't disappoint. For only 99 cents, it's a great pick-up-and-play download for both die-hard fans of the franchise and anyone just looking for some side-scrolling action.