Virtuemart is highly configurable and powerful.We have had it running 12,000 products over 766 categories with ease. You can stock the system via CSV in minutes and export data into all major accounting packages. We have had it configured to recieve realtime stock information from shop floor POS systems.
The real beauty of this extension is the amount of people developing for it. Including us. Search google and you will find pretty much anything you need. Clients love the administration as once configured it is a breeze for them to use.
Do not under estimate virtuemart. We still do not understand why this software is free but glad it it is.