IMO, there are three basic ways to use a GPS on a bushwack:
1a) Follow someone else's track. While the travel may be rougher, navigation-wise it is little different from following a trail.
1b) Make your own track at home and follow it. (Ie the same as 1a except for the source of the track.) As peakbagger noted, there are things on the ground that may not show (of may not be noticed) on maps...
2) Use the GPS to provide headings to some intermediate and/or final pre-planned waypoints and travel between the waypoints using the path of least resistance. This is essentially the same as traditional non-GPS bushwacking except that one can include electronic reference points and can be combined with non-GPS methods.
3) Turn the GPS on at the trailhead, reset the trip computer, throw it into your pack and leave it there until you get back to the car where you check the trip computer. You can think of the GPS as a super pedometer... However, it is a form of safety gear and you may be bolder because you know you have a navigational backup it your pack. (Also known as carrying your security in your rucksack...

) After you get home it can also be nice to check your tracklog so you can see where you actually went.
I personally generally use 3), rarely look at the GPS while hiking, and finally check the trip computer to see how far and fast I/we went. If the route was unusually interesting I may check the tracklog, but often don't bother (particularly if the hike was on mapped trails).