At work we're running out of IPv4 addresses and there's no chance that we can get more. Most likely we'll use NAT, but we're investigating doing this the "right" way with IPv6. Most of the traffic from the proposed IPv6-only boxes would be to internal or IPv4-only global sites, which our internal router would bridge, so the actual traffic through H.E. would be only to IPv6 global sites, i.e. very little traffic. Nonetheless this would be a production environment: two computer labs, two Beowulf clusters, and grad students' desktop machines.
Would this be considered fair use of the H.E. tunnel service?
Of course the ideal would be to convince our Campus Network Services to provide IPv6 connectivity.
That seems like perfectly reasonable usage. You want IPv6 connectivity, can't get native yet, so you use a tunnel.
Just remember there is no SLA with the free service.