I've used the Instinct cart a few times and my clients seem to grasp it very well, as far as adding and editing products.
The one thing I HATE, is how it deals with customer registrations. I'm actually in the thinking stages right now of creating a plugin that will manage customers a bit better.
If you want people to have accounts, then they have to register. The only way they can register is through the WordPress registration. Which completely sucks...because then WordPress has to send them a password, and then they have to log in with that password and change it to something else (IF they know how). When they're logged in to the wordpress Admin panel, they see all kinds of news about the latest wordpress...which is confusing and not user friendly at all.
Aside from that - the cart is easy to use, easy to theme...there are a LOT of WordPress themes out there so that's one of the reasons I use it, because of how well it ties into WordPress.
Another option you may want to look at is Shopp. Costs a little bit of $, but solves that customer registration issue. Haven't used it yet, but it looks nice.