Not worrying about the actual technology that could do this, one can look at the maximal theoretical efficiency of interstellar travel: you could draw on your rest mass until your ship was essentially converted into a light beam of the same total energy. This would arrive nearly instantaneously at any destination in the universe. But you can't do this because you also need to conserve momentum. So the best you can do is have half your energy go into a light beam in the opposite direction. So there's the ultimate efficiency right there, not worrying about how you do it. You convert half the rest mass of your ship into light going one way, and half going the other. Then you go with the half going the desired way (as light energy, of course) until you get to your destination and reconstitute the energy of your ship back into rest mass (and you). There you are, no problem with energy or momentum, but you only have half a ship left. In your own rest frame, you might not experience anything all that bizarre, if you do this all gradually enough to keep the g forces down.