I'd like to know what everyone's experiences are with various shopping cart solutions. I'm considering Google Checkout for payment processing, but would like to know of alternatives as well, and any reasons for or against those alternatives. At this point, teh technology stack doesn't really matter - but please specify what stack is being used if you suggest a solution.
clarification: I'm interested primarily in your experience with shopping cart solutions (pre-built). I'm also interested in the payment processor (paypal, google checkout, etc..)
One specific question though - what kind of integration with UPS is offered? I'd like for the user to get an accurate estimate from UPS and for the administrator to have the ability to print a shipping label.
Bottom line - I am looking to get something up and running pretty quickly, and would like to know what your experiences have been.
Thanks,
Ian