I once had a client that I built a Yahoo store for.
I really did not like it that much at all.
You're very limited in what you can do with the layout, design etc.
Now on the other hand, I have also had some great clients that I have built some great shopping carts for
http://www.bestfilters.com/ is my fav.
It's about 18 months old.
Im working on one in ASP.NET