I came to know about Magento before Ubercart. But as ubercart is a drupal based module where you can install other modules like forum, blogs, etc without a problem and handling is so easy that I personally prefer Ubercart in front of any other shopping cart including paid ones. The most important benefit that you get from ubercart is of SEO (title, headings, url, descriptions are in the way the search engine requires), easy navigation and off course valid XHTML. Changing themes (design) of your website is easy.

I give Ubercart 9/10 (including all open source, free and paid shopping cart).

If you want to use the best SEO shopping cart then Virtuemart and E-commerce module are not near to Ubercart. If ubercart doesn't stop its continuing effort to make it more better and better, I think it will be one of the most sought shopping cart in coming years and will eventually get the maximum market share.