While the pickings are pretty slim for iOS 7 controllers that are actually worth buying, things are getting pretty awesome on the software side of things as more and more games that were originally made to be played with controllers slowly begin to get controller support. Sonic the Hedgehog 2 [$2.99] already had controller support, but last night both Sonic the Hedgehog [$2.99] and Sonic CD [$2.99] joined the controller party.
I'm not sure if the Sonic games specifically are worth picking up a controller for, but if you've already got one, the Sonic games are definitely worth playing with proper controls. Additionally, the Sonic games are just worth having anyway, regardless of whether or not you've got a controller to play them with. The love they've gotten by Christian Whitehead and Sega is really remarkable.