I use and am partial to CS-Cart myself. It's not free tho like osCommerce is. I started out with osCommerce & moved it because it wasn't working for me.
First off, most of the important things, such as SEO, need to be added to osCommerce yourself. There IS an extensive list of modules you can add & all come with instructions, plus they have support for them in the forum. BUT the more you modify your store, the harder it is to add additional modifications. All instructions are based on unmodified code. Using a comparing editor helps but you can still run into problems.
CS-cart on the other hand comes with everything I wanted to add to my osCommerce cart (& I am an anal perfectionist & wanted everything!).
Cost is about $200 tho, but well worth it (for me anyway).
That's my 2-cents.
