ProCam is good, but not quite as handy IMHO as Camera Awesome! from SmugMug. But hey, try them both.
Of all the apps that let you adjust an image after taking, Snapseed is the most useful, better even than Photoshop Express, because it's so easy to use as well as delivering great results.
These two apps let me document a three-week vacation entirely with my iPad, in near real time via Flickr. No, the photos and videos are not professional quality. But they're good enough to show off the Retina advantage over prints when I share the trip with friends. Waterfalls were made for HD video! If you're hiking, a padded case/bag with a shoulder strap is a really good idea.
Then there's Old Fart Booth, which must be tried to be believed. Warning: DO NOT use it on women without obtaining informed consent first.
milliHelen: amount of beauty required to launch one ship.