Some good points raised however GPS has been around some time now.
Most decent mobile phones have good GPS and a decent map installed.
My N95 has no trouble locking onto a signal in the aircraft and showing where I am, if this is all you are after do you need to purchase anything else?
It even tracks my flight for later download and can show height/speed heading etc.
Not saying you should use this as a dedicated nav tool but if you only want something as a basic tool to assist your nav then it will suffice.