I can understand why some reviewer give low rating. Because virtuemart is not 'out of the box' shopping cart extension.
With TONS of features and flexibility, it will be difficult for VM team to create one solution that fits all online business. So when you install, you get the GENERIC template which simply need lots of tweak.
But in developers perspective, when you able to understand Joomla MVC, VM2 is really a complete shop that has TRUE integration with joomla, you can do almost anything.. the sky is the limit !
Why i don't give 5 star ? because of the documentation! At this time of writing, this is where Virtuemart Team need to put more commitment, even their wiki for Virtuemart 2 plugin development is outdated. (per December 2012) This can mislead a lot of people who wanted to develop solution with Virtuemart 2. So, if you want to learn VM2, you must gather information yourself in forum and by compare the code from default plugin.
Conclusion ? Virtuemart 2 is in the right direction to build powerful shopping cart in Joomla, it need extra work on the documentation (i think you should give the user guide for free instead of buying it from oscmanuals).
If you claimed to be a Joomla developer, this extension is worth 'time investment'
Thank you for your comments re: documentation, it has been noted.
The support even better, i purchase several commercial extension and csvi pro is the top leader in support service. Every problem post will have feedback within 1-5 hours, Roland explain a lot of detail clearly, he know what he did.
Hopefully this excellence support and product continues. I recommend this extension to all Virtuemart Developer. A must have !