Google has done a good job of describing the product in detail at the following URL:
http://webaccelerator.google.com/support.html

The Google Web Accelerator is currently in beta test mode. As Google states, it is intended for broadband connections - "Dial-up users may not see much improvement. Optimized to speed up web page loading for broadband connections." Following our internal testing we have concluded that the architecture does indeed appear well suited for high-speed broadband, but not for those accessing over lower bandwidth connections. In addition, Google's Web Accelerator and SlipStream's solution cannot co-exist. During our tests, Google was found to essentially slow down the browsing experience over dialup by between 13 and 28% on average. In one case it was actually 87% slower. The latest independent test results conducted by VeriTest concluded that SlipStream improved performance by up to 7x. We have also tested the Google Solution on 128Kbs and found no significant improvement. However, SlipStream significantly accelerated browsing over this connection speed even with original image quality.

In order to achieve the broadband performance gains, the Google Web Accelerator loads the content of some web pages in advance (pre-fetching), caches copies of an end user's viewed pages, and compresses data before it is sent to the subscriber's computer. This architecture is well suited to high-speed broadband connections where the bandwidth capacity is available to handle this additional data caused by pre-fetching. It has serious limitations though for lower bandwidth environments, including dialup and shared bandwidth access such as cable, WiFi, ISDN, cellular, etc. The pre-fetching of data means more bytes are being transferred than would normally be the case for the end user. Thus, in situations where one is paying for data transferred, the usage charges will in fact increase.

You should be aware that users are expressing concerns that this Google service can cache more customer browsing data than they would prefer, and this in turn has raised concerns about how this information could be used.

We have also completed initial testing of the beta product to see what happens if both the SlipStream Web Accelerator and the Google Web Accelerator are installed on the same customer computer. We have determined that the SlipStream solution will always remain active and the Google Web Accelerator will not be engaged. It could be confusing to an end user, who may incorrectly conclude that the Google Accelerator is delivering the acceleration when it is actually SlipStream. It will be evident that SlipStream is doing the acceleration because the SlipStream icon will indicate activity by flashing and the Google icon will not be moving. If someone wanted to run the Google Web Accelerator, they would have to stop the SlipStream Web Accelerator or exit from the SlipStream application.

In summary, the Google Web Accelerator is optimized to speed up web page loading for high-speed broadband and will slow down browsing performance on dialup, lower-speed broadband, and cellular connections.