Like a few others, I've used Snapseed effectively on a Mk 1 iPad (and on my iPhone 4). I did not find the lack of a Retina display to be an issue. It's good for straightforward PP, including sharpening, improving saturation, brightness and contrast, and for cropping. Some of the effects filters are also useful (Drama, BW conversion). I still rely on LR 4.4 on my Mac for more robust PP, e.g. use of brushes, finer tuning of color channels, etc, but the iPad+Snapseed combo is hard to beat when I am away from the house (or when I am just too lazy to boot up the computer). I used to use the iPad SD card connector to download, but have lately started using Eye-Fi SD cards to transfer directly from camera via Bluetooth.
I also have PS Express loaded on my iPad but have never really tried it; I also recently got Superimpose, but it is more of a masking/photo montage tool. I've used this app for doing goofy things with pictures of my boys.