I have found Google Analytics to be way off on ecommerce conversion tracking. It always reports way less sales than we have, both credit card sales and Paypal. For instance, the last period from 5-26-09 to 6-25-09 reports 44 sales when in fact we have had 110 sales. I cannot figure out why some transactions register with them and some don't, there doesn't seem to be a pattern that I can discern, anyway.
So, with this metric being so off, I am wary of any other analytics data being accurate. I am to the point where I am going to dump it and save the 20-something KB of bandwidth per page load that the GA script uses.
I'd be interested in hearing of any other analytics out there if someone has used them.