Paypal is by far the most widely used payment method worldwide.
So if you don't accept it you may miss out on some sales.
(But like everyone the 21day holds and the suspensions for no reason worry me/us)
As to which is the next most widely used - who knows ? - we'd all like to know, but I have not seen any figures.
Google Checkout cannot be going that good or they would have extended it to many other countries by now.
I think Google just play with some things eg Google product Search and Google Checkout..