PS express is a mini version of Photoshop. On my phone I can edit brightness, saturation, contrast, etc and add a few simple effects such as black & white, or blurred edges to the photo. I'd be willing to bet the iPad has a bit more functionality.
TiltShiftGen lets you adjust exposure, similar to pse, but gives you more choices when you want to blur an area of your photo. (square area focused, or round, oval, etc) To me it's worth getting despite the duplicate functionality.
CameraBag crosses over from "make the photo look nicer" to "change the style of the photo". There are about 15 modes, like an old instant photo, a fisheye lens, a photo taken in 1972, etc. It's nice variety without an overwhelming number of choices, and while you're deciding which one to apply to a photo you can easily slide from one style to the next without having to save or reopen the photo.
The next two, PhotoStudio and Lo-mob, give you a WIDE range of choices. PhotoStudio even has an option I love - click on the dice cube and get a random effect applied to your photo (there are over 200 categorized effects to choose from! Very fun app). This has the most gimmicks (some are just weird, honestly, like overlays of butterflies or a skull or flames...)
Lo-mob doesn't have quite as many tricks but the cool thing is that most effects mimic an old style camera (with viewfinder marks visible, etc) - it gives a nice "we found this photo in the attic" vibe.
The last app, Dynamic light, is for the HDR effect that people tend to love or hate. It boosts up the range of tones in your pictures (compare the visual difference to playing a folk guitar and then picking up an electric guitar with a big amp). This app actually lets you dial how much HDR to add and gives you various HDR styles to pick from.
Great for scenery - give those blah photos a bit more punch.
Well, there is my highly scientific analysis... Feel free to ask questions. And remember, if you actually have a camera/smartphone there are a bunch of other apps that take a photo and then do great things. (it's normal for an app to let you take an existing photo from your library or shoot a new one. Most of the apps I mentioned do that. Only Hipstamatic doesn't let you edit a photo you already have saved on the phone.)