Sonic CD is an excellent iOS port of what's widely considered the best game in the Sonic the Hedgehog franchise, an ill-fated but critically acclaimed 1993 release for the Sega CD console.
Anyone who has even passing familiarity with video game history will know the Sonic schtick--speeding around, spinning and jumping, and collecting rings--and the coolest thing that sets Sonic CD apart from other Sonic titles is a time-traveling feature that essentially lets you experience three different versions of each level (past, present, and future). The game looks and feels like the original, with settings and unlockable extras that Sonic fans will love--like the ability to switch between the Japanese and U.S. soundtracks and the playable addition of Sonic's sidekick Tails (who wasn't even in the original) once you beat the game.
If you love Sonic games, Sonic CD is a must-download: to experience a piece of gaming history, to reward such a well-done port, and just to have fun with such a great-looking, fast-playing arcade game. If you find the iPhone or iPod Touch controls lacking, you'll probably find the iPad's wider virtual touch pad a bit easier to master.