Interesting, although it's 10.4 only so no good to me at present. Ars Technica have a review [arstechnica.com] which tries to answer the obvious question about why you'd want this since you already have Sportlight and Quicksilver on this platform.
As mentioned, the Google search box gives me results noticeably faster than using Spotlight, but about the same speed as Quicksilver. But unlike Quicksilver, Google Desktop doesn't currently offer any of the extra functionality or that make using Quicksilver so handy, such as (for example) being able to perform a search for a specific photo and then instructing Quicksilver to automatically upload it to Flickr. "Quicksilver does offer some functionality that Google Desktops doesn't," said Yao. "But Google Desktops also offers functionality that Quicksilver doesn't. All of these tools are complimentary to each other."