I know that some people have had issues but from a reliability point of view, Fallout 4 has been the best yet.
I've not had a single crash or stutter, and I get consistently good frame rates (despite Nvidia GeForce Experience claiming that my system doesn't meet the minimum specs!)
I have obviously seen the odd bug, there are certainly some graphics issues with clipping, but all in all it's a pretty good experience.
I have left the game running for days, just pausing to menu and alt-tabbing out when I've something else to do (like sleep!)
One thing I have noticed when starting up is that Steam seems to be doing a lot of updating of the game, although there's not been an announcement of any product updates? Dunno what that's about...