There are lots of apps that let you add text to your photos. Over HD (US$1.99, but currently on sale for 50 percent off) does it with grace and style.
Pick or take a photo, open it in Over and add your text. The font choices are very artistic, not just the standard off-the-shelf variety. You can move the text around, change the size and reduce the brightness of the background image. Then you can share your photo through the usual social tools and email. There is also an option to have your photo delivered as a real postcard, at a small cost of course.
The rendered fonts are of very high quality, and the GUI is best in class. This app would be my first choice to add text to my photos. You get 25 fonts with the app, and for another dollar, you can add more. I think the default fonts are varied enough to not make me want to purchase additional ones.
It's not, however, as full-featured as I would like. You can't add multiple labels, and you can't rotate the text or create a drop shadow. I'd like to see those functions added.
If you are looking for something free, check out Phonto. It's not as pretty and the fonts aren't as cool, but you can freely rotate and add additional text labels. The app is ad-supported, but for an additional $0.99 you can have an ad-free version.
Still, Over HD is my favorite way to add text to photos on the fly. It is easy to use, has a beautiful GUI and the text looks just great.
Over HD is a universal app, and is optimized for the iPhone 5. It requires iOS 5.1 or greater.