I can honestly say that I have been much happier and more content since I started using GTD than before it. The main reason for this is that I can truly enjoy my time away from my job without the niggling worry that I have forgotten to do something. This is my biggest win by far from GTD, but there have been many, many others as well.
In terms of the task switching, I often find (like Barb) that I don't really switch as often as you might think. I will regularly start with one action but continue to work on the task and get through more than that single action that I started with - the bookmark analogy is perfect for the way that I use my lists. Some days switching between tasks can be good for me, particularly if the original task is quite intensive, as it gives me a break.
I must admit that I have never looked at GTD and thought that it was suggesting that you should only do the action you have defined and then stop and move onto something else...