Freemium has earned kind of a bad name on the iPhone -- a lot of people see it as a nickle-and-dime kind of thing, where if you don't pay out a buck or two to the game every once in a while, you don't get to have a full-featured experience. But there is a way to do it right: have a full game before the microtransactions, and then use those transactions to make the experience better, in an optional way. That's exactly what Gun Bros does. The free game that you get on the App Store is a more than excellent dual-stick shooter with some really nice RPG and social elements added into the mix. And even without spending any money, you can have a fun and full experience playing the game.
You probably don't need me to explain what a dual-stick shooter is by now, but Gun Bros' RPG elements really make the game stand out. You earn XP as you play, and that XP helps you build up and customize your own character over time (one of two bros, Percy and Francis Gun). You also get another currency, and that's where the microtransactions come in; you earn explodium rather than gold as you fight, and that explodium can be "refined" into gold by playing the microtransaction game. You can either convert the gold instantly for free, or you can pay a little more to convert the gold over time, earning more than you usually would. It's a nice temptation, but you don't really need it. All you'd be paying for is time, and if you're patient and enjoy the game, you've got plenty.
There are also some cool social elements: the game is Game Center-enabled, and you can actually bring your friends' other brother in to play with you asynchronously. They don't actually play with you, but your brother in the game will have all of your friends' power-ups and XP, and they'll even get rewarded with XP after you're done playing. It's a cool idea, and it adds a nice social feature to the mix.
Unfortunately, while the aesthetic of the game is awesome, I did see quite a bit of slowdown as I played, even on my iPhone 4, so the game isn't quite as optimized as it could be. Still, it's a lot of fun, and the price is definitely worth it. You can download Gun Bros from the App Store right now.