VN-Tag is a great technology. When coupled with passthrough, it takes virtual switch completely out of the picture. However, hardware VN-Tag will cost more per VM since a virtual port costs only a small amount of physical memory. Furthermore, passthrough requires guest VM memory to be locked, killing memory over commit and consolidation ratio. So I think VN-Tag is a niche technology at best. It might make sense to run VN-Tag alongside Swordfish, where only I/O intensive workloads are put on VN-Tag.
Moving forward, the question is which implementation provides a better foundation for multi-tenancy and scalability in the cloud environment. Only time will tell.