Tau-Zero by Poul Anderson explores this, BTW. It's a great little sci fi novel about a spaceship doomed to accelerate at 1G indefinitely.

It is significant from a colonisation PoV. With sufficient acceleration capability and the ability to survive travelling through the interstellar medium at extreme velocity (rather than getting vaporised by a mote of dust), a single generation of humans could in theory colonise the whole galaxy within their own lifespans. Indeed, some of them could even come back together and meet again after visiting those distant worlds, on an Earth many millions of years older, if their worldlines end up with similar proper times.