I am using Miva for one site and AbleCommerce for another. I think they are both great products with loads of features. Miva is often included with hosting plans that run as little as $25/month. Able Commerce is a little steeper in cost, but a little more robust. Using Miva, I had a store up and running in less than 15 hours of work. Search on Google to find either of these. Unfortunately, I don't know much about Storefront 5.0.
There is a Storefront newgroup that you can solicit feedback from. Its fairly active. You can find a link to it at www.storefront.net I have been using the storefront solution since 99 and have been fairly happy with it.