The time increase depends a LOT on who is playing. But probably double a normal game. I think the fun payoff of the extra people is worth it, though. There are still only one each of longest road and largest army, with two more people, so those tend to get stolen a bit more.
I suppose the increase in shoreline is not quite in direct proportion to the increase in acreage, but I don't think ports are dramatically different. You add real estate to make up for the added players, so the overall ease of getting good position is about the same, I think.
Building in-between turns is probably the biggest contributor to the game going longer, since you have an extra phase to do it in. That said, if everybody knows what they want it can go pretty fast. And yeah, it can save you from the robber, although IMO the robber would be way overpowered if you couldn't do it.
I think that 5-6 is the most fun when you do it with Seafarers, perhaps the scenario with the fog-of-war, since now you can have everybody exploring in different directions. Pretty cool.