So why should we use Google Checkout?
The advantages for offering Google checkout used to be clear: lower transaction fees and you could offset any fees against Google Adwords. This made up for the fact you had to spend longer on integration, had less flexibility, had to purchase an SSL certificate (Paypal you don't) and you split payments across multiple processors.
Now the pricing is the same as Paypal (and, taking into account cross-border fees, is actually more expensive then Paypal! Bad luck for any UK sellers selling to Europe or Eire!), there appears to be absolutely no benefit to offer Google Checkout: in fact, if you drop it and just offer Paypal (which has a larger market penetration, easier to use and offers a variety of integration methods), you can then take into account Paypal's discount fees (which Google have exactly matched).
Some people have expressed on my blog at http://blog.rac.me.uk/2009/03/11/google-checkout-same-fees-as-paypal-what-advantages/ the fact they think Google is trying to "kill off checkout": it hasn't been a huge success, hasn't launched in all countries, and it appears to have no benefits over other services now.