Have used Osc commerce on over 30 sites after trying several paid scripts.
The support is instantaneous through their forums, and nothing that happens to you has not happened before.
Osc can handle affiliates, multi-shops,multi admins and an endless amount of configurations.
At last count there was over 1800 contributions and 80 different credit modules. The whole focus of Osc is to become more and more moduler so that the custimization becomes a snap in.
The best way of seeing what it can do is go to the contributions page and have a look at the variations available.
http://www.oscommerce.com/community/contributions
It is a matter of working out exactly what you want to acheive and then back-up, back-up, back-up.