I have an iPhone 5S with Sky Demon and it's my primary go to GPS so yes it's brilliant. Unobtrusive and faultless in operation. Never had any problems with it at all. You don't need to be on www. for it to operate, it works off the inbuilt GPS. I don't bother turning the GRPS off on mine as it will live update winds etc. I reckon it's good for about two hours min on the internal battery. I've never had it run out but on a recent 1.50 flight it had got down to around 45% on the battery. Of course you can plug into the a/c fag lighter if you have one.
I also have an iPad mini running SD but TBH it's much less hassle just to sucker the iPhone onto the sidewindow and of course the phone is a lot smaller. At the end of the flight you just shove it in your pocket and it's your phone again.