We have mostly all played Tower Defense games like Plants VS Zombies or DoTA. Kingdom Rush is a game similar in nature but has its own spin on the genre. One thing I really latched on to was the unique art style of the game. Another rad thing it does is allow you to upgrade the kinds of towers you want to use. Many bad guys require certain towers to kill, but if you want focus your play style some, you can.
The game is also challenging. Its not frustrating but it will challenge you. I love Plants VS Zombies but really didn't feel like it was very challenging. In Kingdom Rush you can't stick with the same strategy you must adapt to new bad guys the game tosses your way. I guess ultimately Kingdom Rush does very similar things that most other tower defense games it just does it so well it stands above some others ive played.
If I ever get an iPad I will pick this game up but until then I can play it for free in my browser. Yay!
http://www.kingdomrush.com

Yeah, I lost an evening to this game a few months back (i.e. pre-Diablo 3) and have to agree. It's a lot of fun. It's also not soft on the strategy, you can mess yourself up badly with the wrong upgrade tapping your cash at the wrong time. WAY FUN!
It really is. I plan on coming back at it when I ever an iPad.