I spent WAY more time time getting Google Checkout running than I did getting Paypal running. Google Checkout is way more complicated and takes much more code. With Paypal IPN I get one http post (their "IPN" - instant payment notification) with all the info, ack it, and I'm done. Optionally, I can "post back" to them to get it "verified". With Google checkout I get eight posts, although fortunately I can ignore five of them. The three which I cannot ignore are (1) the new-order-notification, (2) a callback to which I send sales tax or VAT and (3) a charge-amount-notification.
Google Checkout is a fundamentally different service than Paypal. It is more like the old Microsoft Wallet. The idea is that Google Checkout "protects" customers from supposedly evil merchants. Google Checkout often sends me an email alias instead of the customer's real email address, and has some ridiculous policies about what information I collect and when. Fortunately, I don't need nor want much customer information. More important is the fact that the customer must create a Google Checkout account in order to purchase my product. This is not true with Paypal (although if the customer does have a Paypal account, Paypal gently twists their arm to make them to use it.) Although I haven't studied the stats in detail, I often see messages from my server indicating that customers choose one service, then go back and choose the other. Ultimately, about 80% choose Paypal.
Besides these differences, I have the following gripes with Google Checkout:
(1) I get a payout from them every business day. So, to keep my checkbook balanced, I have to enter 20 variable payments every month. Amazingly, there is no option for less frequent payouts.
(2) Nowhere in any of those eight notifications do they tell me what their fee is. So, at the end of the year I'm going to have to do some database queries to manually deduct Google fees from my net income on each purchase. Google engineers have entered a bug to fix this, with no ETA. I cannot understand how supposedly some of the smartest people in the business could have overlooked something so basic.
(3) The Google Checkout Perl modules had several bugs and had not been updated to support new features in a year or more, although recently one of their engineers asked me for my patches, so maybe they'll update them soon.
Finally, my server needs an SSL certificate for Google Checkout. This was not necessary for Paypal Standard.
Google Checkout may be well-designed for merchants who are comfortable farming out their entire sales, advertising, accounting, databasing and customer service over to the Big G.
For one, I just don't want to do that. It's probably a personality thing. I didn't get into this business to become a franchisee of a large corporation.
Also, my main reason for adding the Google alternative was in case Paypal ever had a technical issue, as I had with eSellerate. Suddenly, in April 2006, half the customers who wanted to buy Bookdog got no response from esellerate's server. Their tech support and my account rep had no clue. During the next frenzied week and a half, I learned Perl, threw together a keygen algorithm and got Paypal going, while reading angry emails and filling orders manually. It was not a fun time.
I spent a few minutes looking at Amazon's payment processing service and it seems more Paypal-like. If I had it to do over again, I would look at adding Amazon as an alternative to Paypal, instead of Google Checkout.
Jerry Krinock