The gameplay in Major Mayhem involves your character running to cover and then you use the mouse to shoot enemies with a variety of weapons when they pop up. It's a simple premise, but a great game to play when you've got a few minutes here and there. It's a mobile port, but unlike many ports I've played the interface and controls work great.
The game is a little short, but still long enough to be worth the $5 it costs, and it has a few other game modes and a ton of achievements (that involve a lot of grinding) to keep you coming back.
Finally, I noticed a couple of typos while playing last night and sent a message to the Major Mayhem Facebook page. I received a response before I woke up this morning saying they'll be fixing those in the next update. Great customer service!
I managed to get a copy of the game in a bundle for less than the $5 asked here, but I enjoyed it enough that I bought an extra copy at full price to give away to a friend. You should probably check this game out!