Yes, I was able to use Cartweaver alongside Silverstripe. It's not integrated - just hardcoded a link to the Store on each of my page types. So my clients have a backend for Silverstripe and a backend for the Storefront - but I've heard no complaints whatsoever. And since I purchased the PHP version of Cartweaver, visitors to the site can't tell that the Storefront isn't integrated - it all looks the same. I just used the same CSS from the frontend on the storefront. To imitate the menus (including 2nd and 3rd level) I just went to "view source" from the silverstripe site once I had my subpages created and copied the menu section. Then I hardcoded those menus into each page in Cartweaver. The only drawback is down the road if my menus and submenus change in Silverstripe, I'll have to update each page again in Cartweaver - but that takes less than 5 minutes to update and upload the changes.
Let me know if you need further explanation. I am extremely thrilled about Cartweaver. The online community support is terrific and the functionality is exceptional. No complaints whatsoever.