We used Google Checkout and had no problems at all. The only thing we found a bit of a pain was sometimes it took too long for the transactions to be authorized and therefore orders we wanted to ship same day did not ship until the next day.
We stopped using GC when they increased their rates.
Hi, we use Google checkout for a couple of our sites and have found it fine!
Sometimes there is a delay when they are carrying out a check but I guess this is for your own safety. The support is also a bit frustrating, there is no phone number and you can't detail the problem on their contact form but we have only had to use it once when we were setting up.
Customers don't seem to have any probs using it and we've had no real problems to date.