You're very misinformed my friend. One of my clients websites uses Zen Cart (which is a modified version of OsC) and has over 200 users at any given time of day and over $30 000 sales every month.
It's not about how many users or sales you have every month, because you can have above probably with one page and paypal account without any backend. Well, we had it on Ebay without any website.
Just because it's free it does not mean it's not meant for a serious business. Zen Cart and OsC are by far one of the most advanced shopping carts available.
I said, that when we review it, it didn't meet our requirements. So, nothing against open source. Besides, company which developed website for us, also provided all backend software free of charge. Actually the reason I wonder if we pay to much and post on this forum, is because I can't figure out "what's the catch", since even from Elance.com we got $30.000 lowest bid for our project. Now imagine what web agenesis from high street asked us.
I only have experience with Zen Cart but their functionality is nearly identical. You can track customers, items, inventory, sales reports, products viewed reports, export to quickbooks, print invoices, manage clients, bulk e-mail and lots more.
The only time I would imagine you would need a more powerful shopping cart is if you run a very very large scale store such as Amazon.
Out of curiosity, what were you looking for in a POS system that OsC did not have?
We were looking for something like Netsuite < http://www.netsuite.com/>, but with better catalogue and CMS capabilities and on our server ( not as ASP). OsC POS is basically is a single sale channel system ( shopping cart), and perhaps it's very good and easy, but we needed multi channel system, because our pricing, availability and sale terms greatly depends on where, when and to whom we sell.
Anyway, thank you for your comments.