The service is a lot like a CDN, content delivery network, where Google will fetch your pages, optimize them, speed them up and then deliver them to your end users. They promise speed improvements of up to 65%, but typically between 25% and 65%.
If you want to try it out, you can request to be a beta tester on this form. But keep in mind, Google will eventually start to charge for the service - plus they aren't yet disclosing how much. So if you think you can't afford it, then maybe don't bother. Google does promise the "pricing will be competitive" and they will give you 30 days to decide once they announce the pricing.
A WebmasterWorld has discussion around the service. Some are saying they gave it a try already and it has resulted in making their site slower! One said, "I ran the test and my original is faster than the optimized version." Many add they don't want Google to do this for them and they won't even try it.
I am curious, would you give this a try?
Forum discussion at WebmasterWorld.