I'm a very heavy GPS user during sales trips as well as back country trips. A device that does one very well is often the antithesis of the other purpose.
What I want in a driving GPS:
Big screen
Clear speakers
Toll Avoidance / remap around detours
Quality adjustable platform
Lane awareness or at least lane suggestions in driving (exit left in .5 miles)
Acceptable maps
A very large POI database (resturants, gas stations, banks etc)
Updates for POIs.
Acceptable sensitivity (generally high sensitity is wasted here since roads usually have view of the sky).
Thankfully, nearly any of the popular brands even most basic models do all of these things exceptionally well. I picked up an open box Tom Tom for my wife for $60 at REI and it's been fantastic. My old Magellan does most of the same features (minus voice adjustments) and I've had it for five years. I paid a lot more for mine back then, but even the base models are fairly quality.
Things you will not get by using a car gps for backcountry – even if it supports such use:
Swapable batteries
Acceptable battery life (big screen = battery hog)
Quality trail maps
Light weight
Waterproof
High sensitivity receiver that works under heavy cover
Generally it has not been my experience that map updates are critical. Usually by the time the maps are wildly out of date, I've already opted for a newer model. Map updates quickly add up and if you shop around you can sometimes find a new GPS for the cost of two years worth of updates. This is not the case in trail maps, for my handhelds, for those I want a satellite subscription and regular map updates. I've had major trip adjustments because a trail has been remapped.
A good handheld gps is an investment in your adventures. If you are the type to use them, it's best to save up and get what you think you'll need. Personally, I really dig my Oregon 400T. I've written a review on here if you are interested. As a side note, I could have bought five car GPS units for the cost easily.