I have tested three shopping carts so far on my quest to find the holy grail of eCommerce solutions. I was after a preferably free highly configurable (design) solution and I have to say at this point in time, I find OSCommerce the winning solution.
I know many people have slated its inhability to change the layout and design of it but to be honest it is fairly easy. One thing is for certain though - when the boys at OSCommerce were developing and coding it they didnt really have user modification in mind - however u can make layout maintenance in the long run easier by simply adding CSS elements to the existing table layout OSCommerce uses - use table.classes , td.classes etc and you should be on your way to a winner!
Hope this helps anyone considering using it