I really enjoyed Bug Heroes. It's addictive, has tons of content, levelable characters with lots of variety and upgrades, lots of different enemy types, and it is one of the best tower-defense/scrolling shooters I've played on the iPhone.
Featuring multiple upgradeable characters that you can switch to on-the-fly (pun intended), a wide variety of enemy types, terrific controls, and multiple game modes, this is one polished offering that delivers the goods.
With each mode spanning four levels, I can't recommend Bug Heroes enough due to its amazing replay value. If you enjoy dual stick shooters and have a soft spot for tower defense or RPG's, you'll absilutely love Bug Heroes.