So long as you are using a single routing, and an HE tunnel, it should work fine. With that configuration, using radvd for address allocation and routing works fine. It is if you try to do something more advanced, you will feel have problems.
Initially when I started using ipv6 a couple of years ago tomatousb was perfect for my needs. Very little traffic actually used IPv6, and basically I just wanted a tunnel so I could play around with it. Now, I find about 40% of my traffic will actually go through the IPv6 tunnel. As such, I want it to be as mature as my IPv4 connection. So I do want it to play nicely with my dual router/internet connection configuration. I don't want my wife complaining because she can't watch youtube because I'm performing maintenance on one of the routers.
However, most of the problems I experience with IPv6 are not at all problems with tomatoUSB. For example, one huge pain in the neck with IPv6 is if I establish an openvpn connection on my desktop and then start utorrent, about 40% of the traffic will end-up going through the IPv6 tunnel instead. Consequently I do not use IPv6 on machines with vpn.
Instead of a NAT, IPv6 uses limited use addresses. The problem with that is, it makes it much harder to transparently redirect traffic. For example, with IPv4 I can configure my router to redirect different machines through different VPN connections with no changes to the machines themselves. The same can probably be done in IPv6, but it is anything but easy…