All of the descriptions I've seen were "viable" in the sense of "we could hypothetically accomplish this in a few decades a for around $100 trillion". At least that was the estimate for Project Daedalus, which designed a nuclear pulse propulsion probe to do a 50-year flyby of Barnard's Star. Fun to think about but it's going to be a while before even the largest governments can afford to fund a project that most of its creators will never see completed.