It is well thought out, surprisingly powerful and flexible. with the facility to import and export products and to export orders.
The main negative right now is its early stage of development. I only found out about Ecwid by accident. It has quite a low profile judging by Google Trends. As it has only been around a few months (and is still in beta) it doesn't yet support many payment gateways.
In summary Ecwid is well thought out, with good support on the forums - there's even a voting system in place for guiding the future development of the project.
I fully expect Ecwid to be one of the top ten carts of 2010.