Right now it's a companion to a desktop/laptop. I use Shuttersnitch, it's a very handy little app to show customers in a middle of a shoot what you have. Much better than camera's screen anyway.
Then I sometimes use Lightroom Mobile. The problem for me has been that it takes a lot of time to prepare and sync the photos over the cloud to iPad. Usually when I would need Lightroom Mobile the most is when I'm in a hurry (probably traveling), and that's also when I don't have time to wait it to take hours to sync the photoshoot.
I am having high hopes for the iPad Pro. Not only because of the screen size, but also for how Adobe will hopefully see it as a pro tool and release something that is truly pro-grade.