This isn't as straight forward as it first appears. I am a long term user of GPS and have worked as a field applications engineer for a GPS related company.
I have used GPS in the hills for years, Garmins, PocketPC, mobile phones etc and the technology never fails to impress me. A modern well implemented GPS should have no issues with tree cover and most cope with limited sky view and multipath errors admirably. The latest devices with OS mapping are simply stunning devices, the ability to see where you are on a map, often to an accuracy way better than the scale/resolution of the map in question is quite frankly astonishing.
With a bit of care and forward planning dead batteries can be avoided. For example simply carrying spares helps here and ensuring the device is rugged or placed in a rugged case makes them pretty hard to break.
However , for the last couple of years my GPS has taken a back seat, why? because I began to realise by spending too much time looking at the GPS I was missing out on the real reason I was in the hills for in the first place. Going back to map and compass was a joy, by using the best navigational tool you have, your eyes increased my enjoyment. Technology is great but its not the reason I go to the hills.
I now use my GPS alongside my eyes, along side my map and compass. Its another tool, although a pretty good one at that. Actually on most trips I carry two GPS, a small one (dedicated datalogger) that's permanently on and sits in the top of my rucksack to capture a track log of my walk and a second device primarily as a navigation device that's only powered on when needed. That way I always have a complete log captured and always have enough power to navigate if required. Never plot a route on the navigation device, don't really see the point, its time consuming, could be dangerous if you blindy walk to your next waypoint and above all results in you looking at the screen rather than the scenery.
For those against GPS, I say give it a try, marvel in the technology, enjoy the unbelievable accuracy, be aware of its limitations but don't put your map and compass away, it doesnt have to be one or the other, its most fun using both.
For me its not about a GPS being good or bad, its about how it can enhance my time in the hills and I have no doubt that it does just that, but on my terms.