Properly designed they should work quite well. But properly designed they will in likelyhood be more expensive than traditional bed designs. If you are really thinking about building your lathe and want to go with some sort of linear rail design then take a look at this one
or any of the other lathes on the Machine Tool Archives site
There is one that uses round bars for ways that look to be at least 50mm in diameter. Just can't think of the make off the top of my head. Also for a really good bar lathe you need some way to adjust the guide bushings
The optimist says the glass is half full, the pessimist says it's half empty. The paranoid in me says somebody put a hole in it.
Remember pessimists are at heart opptomists. They know things can and will get worse.