In short, Exp-presso Store is the perfect eCommerce midpoint between BrilliantRetail and CartThrob.
In a similar manner to Cart Throb (and Zoo Visitor for that matter), Exp-presso Store uses Channels for its products. To me this is very much "the ExpressionEngine way" of doing things meaning I can use all the power of the exp:channel:entries tag and still choose which image resizing, entry tagging, SEO fieldtypes and addons I want to use on a per project basis, something you can't do with BrilliantRetail which sacrifices this functionality in favour of a more closed and streamlined way of doing things.
Unlike CartThrob however, it doesnt require the same overhead in setting up all the extra fields and settings. Stock, price variations, product options, SKUs, taxes, delivery options, weights, sizes, date based sales and discount codes are all handled through it's single, easy to use fieldtype.
We used it on the Very Delicious Cheesecakes Website we launched just before Christmas. It's simple approach meant we could get an eCommerce site set up and launched on a relatively low budget and short timeframe without losing any of the core functionality the client or customer would expect from an online shop.
Currently the site is only set up for Standard Paypal payments, but the functionality is there for a full payment gateway when the time comes.
The checkout, orders, cart and payment process / tags are well thought out and documented with examples and code snippets provided to get you off the ground. It also placed no significant limitations on how it's used - I turned the otherwise table-tastic basket / checkout set of pages into a purely CSS set of pages that could adapt very nicely to smaller screens and mobile devices.
A special mention also has to go to Adrian and Dave who created the module for the excellent customer service and technical support provided in their forums. It's also the cheapest of the 3 eCommerce modules on offer to ExpressionEngine users which is a nice bonus!
CartThrob and BrilliantRetail definitely have their strengths, but for a simple to set up, flexible and low cost option, Store wins out here. A highly recommended module.