Seen in PCWorld.com:
The Web has long needed a universal checkout process. eBay’s PayPal and other online payment services rely on the various “shopping cart” applications of the Web sites they serve. While browser add-ons such as Siber Systems’ RoboForm can complete much of the order-processing information that different sites require, they exhibit little consistency in the type and quantity of data they collect.
Also, as you place your personal information on more Web servers, your security risk grows. Last but not least, the Web forms themselves are often poorly designed: How many times have you had to reenter all the information in an order form because a single field was skipped or included invalid data?
…
Google Checkout stores your credit card number, mailing address, and other ordering information. You can view all of the orders you place through the service on a single page, and Google limits how much of your information it shares with its vendor partners.
The article does note a potential security risk:
While having a single repository for all your orders makes makes online purchases much faster and simpler (and potentially more secure), Gmail users and other people already registered with a free Google service may have to beef up their security–one log-in name and password opens them all.
Until I signed up for Google Checkout, I didn’t worry much about someone gaining access to my Gmail inbox, because it contains no sensitive data. The first thing I did after adding the Google Checkout information was to change my Google password, and I’ll continue to do so regularly as long as I’m using the service.