Interesting. Of course, their cousin the mag sail remains a truly awesome breaking system, which would drastically drop interstellar fuel requirements. It also seems likely that a system where a ramjet was paired with an antimatter drive would work quite well. Keep the antimatter onboard, collect the matter, and if you mixed them as a ratio of 1 part antimatter to 25 parts matter, you'd get an energy conversion of something like 10X fusion, which might be enough to make the drive considerably more viable.