I am using quick2cart as a multistore-multivendor solution.
It only took one hour to get around the main configuration, this product absolutetly rocks!
Great integration with other extensions.
Excellent presales support, excellent customer support!
For end-users/site administrators
Nice set of features, it compares favourably with much more expensive and heavy solutions. Well thought owner dashboards, quick for managing orders and payments. Check the demo and forum you'll be impressed.
For developers and forward-looking site owners:
The implementation is extremely well thought, the code is clean and easy to understand, change and extend. Nice documentation, nice community and support forum.
I have tried most shops on the JED, I guess this is the absolute winner in terms of maintenance cost, ease of use, social integration. They even documented their APIs, and provide code samples to make our lives easier!