i never venture out in unfamiliar hills, or bad weather in familiar hills without my gps. i use it when i am unsure where i am on the map, mainly in low vis. imo it is an invaluable tool.
if you can afford one, buy one and leave it in your bag (after reading the instructions)
i got caught in a white out caused by strong winds and snow on helvellyn a couple of years ago.my mate had a gps, and we got back to the car by re tracing our gps tracks. god knows how we'd have got back without it. It was a scary predicament to be in with a gps, we picked up 2 winter climbers and a few walkers who were struggling en route.
using a map and compass to get back would have took a LOT longer, maybe taking us into the night, which would not have been good.
i bought a gps shortly after. i know i can find my gr accurately for navigating, and for giving an 100% accurate location to mountain rescue if needed.