I've been using a Bushnell laser for about 5 years in practice, luckily we can also use them in comps now.
The biggest benefit I find is getting an exact yardage to the pin, many yardage markers are way out on a lot of courses, and who needs a 150 marker when you've got about 70-90 yards to go? Added to the fact that we don't always know how far on the green the pin is, the laser is invaluable.
The only times when the laser isn't perfect is when it can't pick up the pin and when you have lay-ups on par 5's. Sometimes if the flag is limp and there are trees behind the green, the laser has nothing to reflect off except the trees so you get a false yardage, but that's easy to work out because you just laser the trees on purpose to compare. On the rare occasion that this happens, just point the laser at the green or bunker and get a distance there, and in any case the GPS can't give you a distance to the pin in this instance anyway so it has no advantage.
On par 5's the GPS can have an advantage as it can give you a distance to a hazard, or lay up area, that's the only time when i think the GPS is sometimes better.
And how about this one: How about banning GPS but only allowing laser rangefinders? Why should you be able to get given a perfect distance to the centre of the green when you are miles from your own fairway with loads of trees in the way? If only laser was allowed, you would only be able to get a distance from something you could actually see with the naked eye, hence a little bit of working out and common sense would also sometimes be required.