I've had a few new propshafts on mine. As above, they're often not much more than a set of new spider bearings and you get a new spline too. If you're just trundling around at 30mph that might not be a problem, but I do a lot of motorway miles so it helps to keep everything running smoothly at speed.
I've had a prop whose bearings had been replaced break up on me whilst driving. Fortunately, it didn't do any harm. What appeared to have given way was one of the yokes that the bearing caps go in. It made me realise just how thin the metal is surrounding the bearing cap, and how, as a casting, it's got a crystalline structure that's rather brittle. I'm now unwilling to trust propshafts that have had bearing caps pressed in and out of them more than once in case the yokes have a hairline crack that's going to give trouble later.