I have just started to work with redSHOP and I think it is very promising. It is new but they work lightning fast with very frequent updates and immediate customer response. You can expect a feature request to be implemented within days or a week! I am not kidding. I have got so much headache from VirtueMart that this new shopping cart seems like a godsend. Sure, there are issues to iron out but I have never seen a team behind a software work faster than these guys and their support is very fast.
I will place the shop in a few RT templates and report the experience.
The redShop is in a way a Virtuemart shop on steroids. It is vast and hace so many settings and options it is overwhelming. The development speed is fantastic with almost weekly updates to include requested features.
The install file is large so make sure you have access to your PHP settings to accomodate the install. After that - prepare to learn, that is what we are doing now for a new project. I will post my findings here as we go.