It's analageous to GPS in cars. Some drivers rely on these devices to the extent that they have no situational awareness, and you get ludicrous situations like someone travelling between two Essex towns 20 miles apart finding themselves passing Northhampton on the M1 because they put the wrong data into the Sat Nav.
Hopefully, pilots aren't that daft, but the device if over-relyed on can and will reduce situational awareness. For VFR cross country flying the GPS is a very useful aid and I always carry one. But I also carry a chart with my track marked on it and that way I actually know where I am (most of the time!).