No, you don’t need a dedicated iPad/iPhone game controller. Let’s just settle that right now. I, and sometimes I still do, but iOS games are too fluid, and too few use button schemes that make sense in a physical controller format.
That being said, there are an awful lot of retro games out there that could benefit from that type of device. But, it needs to be versatile. And it needs to be affordable. Both of these qualities were lacking in the overpriced, incredibly limited, a dual-analog Bluetooth control pad that cost $80 and only worked with a handful of Gameloft titles. Last year's was a fun concept that made for a great desktop toy, but its size, price, and limited game library support makes it largely impractical.
On the other side of the spectrum is the iCade 8-Bitty from ThinkGeek, a throwback-style controller that looks like an escapee from a NES-knock-off factory in 1985. Graced with faux wood-paneling stickers on the sides and a D-pad with four buttons on the front (and two shoulder buttons), the 8-Bitty is a clever Bluetooth controller that does what its larger cousin, the iCade cabinet, did, but with less hardware. It works with a large chunk of the iCade's eclectic, oddball indie and retro supported titles. More importantly, it only costs $30. And, it also works with Android devices. It's an improvement over the iPhone 4/4S-specific released earlier this year.
Which games does the 8-Bitty support? Midway Arcade and Atari Greatest Hits, quirky little games like Forget-Me-Not and Super Crate Box, and dozens more. The list isn’t easy to track down, and the 8-Bitty doesn’t work with all the games the iCade does, but most of them translate well enough. On the Android side of things, the emulator app MAME4droid supports 8-Bitty. Ion’s list of iCade-compatible games gives you a mostly complete sense of the gaming library you can take advantage of.