So I can't rate it highly enough when I think of the previous hassles I had in migrating from Joomla 1.0 to 1.5!!
For all that I'd equally add my warnings and suggestions that you not only ensure you back up your site (including server structure) but also test it out on a test site rather than a live one if at all possible.
However, I do have a couple of minor issues - while there is now the ability to black/whitelist IP addies, there's no easy way to see which are which once you put them in - all that happens is you get a list and as such I've not put my own IP in just in case it gets black instead of whitelisted without me realising. Would be much better if each list was fully shown as what it is.
And that's the only reason I've not given it a 5 star rating as otherwise it really is great but maybe some more work on it would have it truly superb!
I like the frontend which is fine for a small site like mine and in particular loved how easy it was to get it set up with payment options.
I have another site on another platform which doesn't support VM and am having to try to use the much heavier OsCommerce which I can tell you is a TON tougher to set up and work with. And there's almost as little support for it sometimes too, and I agree - nothing more frustrating than not getting an answer at all to a problem, let alone it taking days to do so.
But overall, I would have to agree with others here who've commended the module as I truly feel it's worth the hard work and effort to get it up and running. And given how I've got next to no PHP knowledge or understanding I wouldn't agree that you need to have much - a very basic appreciation of where your modules are stored and how to tweak some basic code lines is about all you might need, if even.