I've tested VM on smaller personal sites for over a year before being comfortable enough to use it on a client site. I set the client up with the FedEx real-time shipping module processing payments through Authorize. The website processes 2.5K - 4K in orders per week. We've had it running for a little over 3 months with no problems whatsoever.
The software required a little hammering out to make it work exactly as we wanted it, but as a non-programmer I was able to get it exactly as the client requested.
As a result, this is the first open source software I've made voluntary donations to. It's an excellent piece of software. Installs easily, completely customizable, and always forward moving.
Install VM and test it out. You won't be disappointed.