I've been examining Virtuemart (VM) for a little over a month now. IMO, the most compelling reasons to use VM is because it is a component of Joomla and easy to setup and use out of the box, that's it. If VM were a stand alone application it would not be very impressive, just another cart, and there are a lot of carts out there that do much more than VM. A lot of the VM code base is not pretty (hard coded HTML mixed in with business logic) so layout customization beyond the basics requires digging into the code. The good part is, there seems to be a lot of interest in VM and if it could ever take full advantage of the Joomla framework and get a good templating engine, it would have real possibilities.