I do not have a personal experience, since I have never used GC so far, but people should be cautious.
Online scams is nothing new, we have all seen those emails from people offering us money or selling us stuff or buying stuff from us. In fact, I personally was contacted by someone who sent a fraudulent check to buy an old car of mine. I didn't end up falling for the scam, but so many people do.
Many people have recently been falling for scams that involve the Google Checkout name. There is an extremely long thread at Google Web Search Help with dozens of people who were scammed out of tens of thousands of dollars through Google Checkout. I should add that not always is the transaction done through Google Checkout, but sometimes it is done through a fake Google Checkout clone (spoofed site).
If someone asks you to pay them for something you do not have yet via Google Checkout, please - I beg you, read the stories in the thread and consider where they have gone wrong.
I also strongly recommend reviewing Google Checkout's security center to learn more about how to prevent being scammed through this service.
Don't fall prey to these people.
Forum discussion at Google Web Search Help.