When I started project I went with GSON as most completed and with a good backing.
I feel now that it is not performing very well. So, let's say when I load array of 200 items (objects) from web service - it will take like 5 seconds to parse it out into object array on my Nexus S. On Emulator it is even more pronounced. In this case I like emulator slowness as it shows all this bad spots very well.
Now that my app is pretty much solid - I'm looking into different ways to do what I need to do and maybe save on install size. I had to bake GSON into my app with custom namespace because of HTC issues.
I've gotten fairly significant performance improvements by switching from GSON to Jackson in past projects.