I have been taking GPS units on planes for the last 5 years. The rules are really set by each airline. Southwest lists it as an approved device in their magazine and has never been a problem using it as any other electronic device. Delta does not include it in their magazine, but it is discussed in their flight manual "Red Book" "Rule 6" as a device that can be used "at all times." But can never be used with an external antenna. I never have used my GPS during the take off and landing phase as I am sure it would be a hassle as most crews do not read the manual! but if they do bother me about it I put it away and ask them to refer to the manual. Since I don't argue they usually will look it up when they get a chance.
Although I have flown many other airlines, I do not have enough flights to say what the overall airline policy is. Flight crews are the ultimate decision makers and we have to abide by that. As we educate more crews it will become easier and easier. Many times I have had the stewardess' ask our location.
I take my 60CS and use the belt clip to hook it to the window shade. Much better than my old e-map. It is great to be able to sight cities and know what they are. Your seat mates will ask a lot of questions so be ready. One wanted to know if I could use it to find the baggage claim area! I had to tell him that we couldn't, but he when I told him I could find my car in the parking lot he thought that it might have some real use. LOL