I've used several but the ones I use now for a pure eComm option are ShopSite and Ecommerce Templates. I had used x-Cart sometime ago but no more - far too klunky for my taste. In fact, I opt to build eCommerce integrated into WordPress now - higher value for SEO and easy for clients to maintain.
Ive used ECT as well before but wasnt happy with it performance wise. It was a little slow with the amount of traffic I had. Was a great system for a fair price but I needed to move up. ASPDNSF was the ticket but has went down hill.
Never had a problem with traffic using ECT and it is nice since they have both an .asp and .php version and allows you to use either static or dynamic pages, or a mix. I use it for several clients and they like the ease in adding new products. Price is reasonable and support isn't bad. I do suggest, however, if you get ECT and modify any of the scripts, keep a record of the modifications for when they issue updates, they issue entirely new script pages.
Damn... that's the Achilles's heel isn't it? Most of the solutions I'm aware of make no provision for customizations and upgrades. So if you heavily modify a cart you're stuck with the choice of ignore the upgrade or port your customizations to the new version. I want a third choice - a file where I can place my customizations that won't be overwritten every time the author's update their cart script. Is this too much to ask?
I am moving a current client to bigcommerce It takes a bit to figure out the templates, some areas are out of your control, but overall it is a decent platform and they have an active staff making upgrades based upon user suggestions.
lorax, I should clarify. ECT is designed to be on one level, whether the root level or all in the same sub-directory. Since I use static pages and multiple levels, I had to go into the scripts to change url's, merely by adding a "/" before each one. Otherwise, there is a primary file which is used to change virtually anything you need to from display options to messages. That being said, updates are not a problem as the primary file I mentioned never changes. Regardless, it really isn't that difficult.
We have used a lot of different carts over the years - OSCommerce, Network Solutions, Cherry something, Zen Cart, X-cart, and experimented with many others. After all this, we finally decided we didn't want to be our own administrators but still wanted flexibility. We narrowed it down to Big Commerce and Volusion. I felt they were both equal in term of flexibility, service, and pricing. They both blog and do lots of advice videos. We finally settled on Volusion. The reason why is that you can interact with your inventory using XML - this allows a lot of flexibility if you sell on different marketplaces. It costs us only $99 per month, pretty cheap for a full fledged ecommerce system including hosting, email etc. They rolled out a CDN recently which has sped up the site really nicely.