I use iKnitCounter when knitting from paper patterns/books. I love that you can track as many things as you want simultaneously. Frogging, resetting and and having counters automatically restart for repeats is fantastic. The KnitCompanion versions of these are clunkier IMO.
For virtually all digital patterns I use KnitCompanion. Quick Start is great for the easy ones when all you need to do is count and keep your device awake (which you can't do on iBooks or from Google Drive, where I store my patterns).
I swear, I don't think I would knit anything with a chart if I didn't have KnitCompanion to excerpt them (plus keys and whatnot). Currently I have Starsky (complicated chart IMO) and Ginny's Cardigan (pretty easy chart IMO) on the go and I love it.
I travel a lot on business so being able to carry an endless number patterns with their counters around is awesome. Kinda like a stash of audiobooks!