The three times I've tried to use it I've never been able to pay in the UK using different computers, different cards and different browsers.
It probably works at the moment, but it's still shockingly crap for a google product.
Just search 'Google Checkout not working', the general murmurings is that it breaks a lot. I'd link it but google has also fucked up being able to copy your search results without bleeding personal information. Sigh. They really are turning into a mediocre company.