I tried a couple of them last night and this morning, "Atom Zombie Smasher" and "Zombie Pirates."
Atom Zombie Smasher is kinda a RTS game, with a little bit of tower defense. Each level consists of a city layout which usually has a lot of civilians (yellow dots) wandering around and some arrows along the border indicating where the zombies (purple dots) are going to enter. You're given a selection of troop types/items which you place on the map before the level starts. When it does start the zombies enter and you need to try and fight them off while evacuating as many civilians as possible. You get points every round based on how many civilians you rescue (and how many territories you've completely cleared in the past) and the zombies get points too, i think based on how many civilians they ate or something and how many other territories are still infected. Whoever gets to the end of the victory point track first wins, with various bonuses provided to each side along the way.
The trickiest part is that with the default settings with every level you're randomly given three or four types of troops out of a total of seven, and as far as i can tell some of the types are much better than others. This is amplified by the fact that each type gets experience based on kills and when they level up you can improve their stats. So if you only have the crappy types or types that you haven't leveled up much it can make the map a lot more difficult.
The first time through the game i lost by a significant margin. When i started a new campaign i noticed there were a lot of options, including one to let you choose which troop types you get each mission. The second time i breezed through the game no problem. I'm not sure if that was just because i was able to use my favorite types for every missions and give all the XP to them, or also because i'd learned how to handle things better. I started a third game and did that plus another couple bonus options and the "hardcore mode" option, but i got creamed on that one on the very first map. It seems like balancing the game between too easy and too challenging may be difficult.
Zombie Pirates is also a RTSey game. You control a pirate ship that's fighting the zombie pirates (though really, given the backstory i'm not sure why we're supposed to be sympathizing with the pirates rather than the zombies.) You move the ships around a simple 2D ocean (which eventually gets populated by islands and such on later maps) and have to defeat all the zombie ships moving from the right side of the map to the left. You ship has slightly longer range and a little more HP than the pirate ships, so in order to survive you need the items dropped by the zombies. Explosive barrels to blow up zombie ships before they get within range, glasses of grog which heals your ships, and parrots which let you buy new ships to fight alongside your flagship. Yeah, i dunno why they're using parrots as currency either. There are also occasionally doubloons which pop up and you can collect, but they're supposedly for purchasing bonus stuff which i don't seem to have unlocked yet.
So far it's a pretty simple game, but it's the fun kind of simple. Atom Zombie Smasher seems more addictive, but the weird difficulty curve is a bit of a problem. But Atom Zombie Smasher was definitely worth the $5 i paid for it, and Zombie Pirates was definitely worth the $4.