I met a lady last weekend that had an old eTrex and she said it would lose signal in the woods. I have never lost signal anywhere thank God!
I have an old Garmin eTrex Venture and I loose signals frequently too. Sometimes I think it's the woods, other times I think it's the mountains.
There is one trail up here that is notorious for my GPS not being able to keep a signal on. It is both wooded and surrounded by mountains. My theory is that it takes a certain number of satellites to pinpoint it's location, so if the mountains or trees or whatever are blocking some of the signals it can't pinpoint it's location. I don't know how true that is, but that's my theory anyway!
I do love my GPS, although many times I forget to turn it on at the start of a ride. Or I turn it on and forget to reset the tracking information. But it does give me a lot of confidence to go cross country and explore and be able to find my way back to the trailer. So I really find it a valuable tool. Someday I should get a more modern one though.