I don't think any of them work without "line-of-sight" to the sky. That's what my manual said. I can sometimes get away with having it inside a thin pocket in my shirt or coat but if you can't see the sky (under heavy trees, inside a house) it won't work.
ORIGINAL: etw
I have a GARMIN sitting in a drawer. The thing was "searching" for hours and never did find itself. This was a little over a year from purchase date. I contacted GARMIN, they said they could fix it for $75.00. I paid a little over a hundred for it, can't see plunking another $75 into it. The new basic models, certainly not a GARMIN
are in the $100 dollar range. New, the GARMIN would not work in the woods under an umbrella of trees. Perhaps the new units work better but I just use a compass and topo in new territory. My brother has a Magellan, likes it.