I wish Google Checkout would move into PayPal's market. They've been a monopoly way too long. PayPal has fat margins right now and nobody comes close to beating them.
Google is the right company to compete with PayPal. They could implement it into many of their products and leverage their massive ad network.
Google Checkout competes with Paypal's #2 use case: providing payment processing on third party sites. If you don't need recurring billing and are a US merchant, then it is an acceptable substitute. (I won't say superior, for a variety of reasons -- it is cheaper, free if you use AdWords, but has its own share of headaches.)
Credentials: I sold about $10,000 on each in last 12 months.
Edit: I realize that vaguely saying "It has problems" doesn't help anyone here make decisions. Let me elaborate:
* Google has a Buyer's Remorse feature, which allows a customer to yank back a transaction up to 20 minutes after making it. I don't mind -- I have a 30 day money back guarantee -- but some people probably would. (I also have gotten burned once or twice when people Buyer's Remorse a CD that already got shipped -- I have no convenient way to recall a CD order after it gets sent to the processor's computer. I write that off as a cost of doing business, but it is friction and I hate friction.)
* Google has this "customer friendly" option which it labels as "Opt Out Of Marketing Messages From This Merchant". A more accurate label would be "Hide Your Real Email Address From This Merchant So When You Ask Him For Your Account Details In A Year He Has No Clue Who You Are".
* Google once opted me in to showing a post-transaction upsell (to some sort of environmentalist indulgence -- carbon offset or something) on Earth Day. They included instructions for opting out in an email they sent me, which I did immediately. I wrote a letter On Dead Tree (TM) to express how strongly I feel that they should not make their political statements during the middle of business transactions which I am paying them for. (Any customer who mistakenly clicked the button would find it nearly impossible to navigate back to the software they just paid me for. Guess whose problem that is? Guess who they think is trying to be a pushy git and sell them a carbon offset totally unrelated to my actual product?) If Google wants to interrupt someone's browsing experience to offset a carbon offset, I hear they have some web traffic of their own who they can bother.
* Ironically, their search interface sucks. (Can't search for a customer by name. I kid you not.)
Supposing existing Checkout users convert at higher rates (I have no reason to doubt this -- existing holders of Paypal accounts do, too), there is one problem: there are so freaking few of them. Of the last one thousand people who checked out on my site, exactly _ONE_ had a Checkout account which was opened prior to the day they gave me money.
Guess where they opened that account? Yep. My site, the last time they had purchased from me.
[Edit: I feel Google should be paying me money for hooking so many people on their service, since apparently nobody else is doing the work. Come on, GOOG, "lifetype customer value" mean anything to you?]
The 10% increase in CTR from having the badge in your ad is a big incentive to search marketers. Theoretically that should decrease your click costs 10% or allow your ad to display in a higher position for the same bid.
The 40% conversion rate increase seems dubious, unless it applies only to current G-Checkout users, which as other have said are few and and far between. Amazing with Google's clout and advertiser perks like this their market share is still pitiful.
Google Checkout users click on ads 10% more when the ad displays the Checkout badge and convert 40% more than shoppers who have not used Checkout in the past.
This strictly describes Checkout users' behaviors. It says almost nothing about the net effect of adding Checkout to your ads or site.
For example, the claim that Checkout users "convert 40% more than shoppers who have not used Checkout in the past" is even compatible with the theory that Checkout deters conversions among other users -- leaving the Checkout users' conversions "40% more" only by comparison with a depressed base.
Yet the article headline and opening paragraph change the interpretation to a much stronger claim. Google marketing may be banking on others' sloppy reading comprehension.
(Perhaps Checkout does help, but their marketing material has enough weaselly wording that I have doubts.)
Google is the right company to compete with PayPal. They could implement it into many of their products and leverage their massive ad network.
I've also been waiting for Amazon to move into this market, but the closest they've come is Amazon Payments (https://payments.amazon.com:443/sdui/sdui/index.htm).