We also bought a Garmin Nuvi in August and I've used it many times since then, including a solo trip to Philadelphia. Worked like a charm and I tend to get lost everywhere I go, so I really need a GPS that's easy to operate and use when I'm driving. It is very clear where you should turn, etc. You can also set up pedestrian and bike routes with it. It does "talk" and you can select different accents, even, for it. You can also enter the coordinates if you know them, so if you are doing Geocaching, it's convenient.
You do not have to pay a subscription fee. I'm sure that every few years, they will release an update to the program or something like that. The one that came in my BMW had that -- I bought the car in 2004, and in 2009 I bought an updated CD for the navigation system.
I think some GPS systems cost more than others because of the features. Some of them, I think have voice recognition (you can talk to it and tell it what you want it to find), some have more places than North America, some just say "make a left" and others say "At Main Street, make a left onto Oak Street," etc. So look at the features and decide which ones you want. For example, I don't need to talk to it. I don't mind entering stuff with a touch screen or pushing buttons.
Hope this helps and that your MIL enjoys her new GPS!
-Dorothy