I strongly recommend a laser rangefinder. It is critical to determine how far your clubs will go, then all you have to do is figure out wind, elevation, how hard the green is, etc. I purchased one several years ago (before they were legal for tournament play) to use for practice, but it quit working after about 8 years. It immediately lowered my handicap by several strokes. I now have one with a pin-seeker mode that makes it easy to use and very fast, and I carry a spare battery at all times.
I think that a serious golfer will have some sort of range finder. I don't like the GPS as I want to know how far it is to the flag, not how far it is to the center of the green. I also have a program on my smart phone - but much prefer the laser for it's accuracy. And it is also great to see how far it is to a fairway bunker, or the corner of a dogleg.