The newer ones may indeed work under trees but I would not personally rely on it. Its good practice to get out into the open for a position fix.

The inherent weakness of a GPS with on board maps is that if you loose the unit, you loose your maps.

Don't get me wrong here, I think that built in maps are an excellent idea. But committing yourself to only one source of information strikes me as an excellent way of getting yourself Darwined..

Belt and suspenders time.