As others have said, context matters a lot, but in general I think getting to gameplay quickly has many benefits. You can get a sense of whether something is fun (as you suggest), but I also find that being able to play around often generates new ideas that I would not have come up with otherwise, and early enough that they are possible to implement.
In my game, rather than build a completely separate prototype, I just tried to get to gameplay as soon as possible, and then built out the game from that, but it has a relatively simple structure..