So, Google wants to be the world's web proxy server? I don't think it's such a good idea. Why? Well, start with thinking on why they want it. They want to know more about web users. Much more. If you use GWA, all of your web traffic goes through their servers. They will know everything about you, maybe even more than yourself - because they will have the chance to analyse your behavior.
They say they do prefetching. Firefox does that, and only when appropriate (i.e. when the website uses a special html attribute). They say they compress the data sent. Well-configured web-servers already do that. And the mere functionality of a proxy server, in the past, was the responsibility of ISPs.
Plus, when yesterday I installed it - just to check it out - it killed my Firefox. I had to uninstall GWA, then restart Windows, to be able to use Firefox again. I have no idea why, but I saw another user complaining about it in slashdot (and another one in a blog comment).
I may try it again after, in a few version, Google fixes some bugs and clarify their intentions.
Nathan Weinberg puts it all together on his excellent Blog News Channel.