Yup, disappointingly short.
So, finished first run in about 5.5 hours, with "some" messing around. I'm well into my second run, and I'm doing a lot more exploring, and I have about 4 hours in and am about half way into the main quest. I say again: If you like exploring, it's going to take quite some time to see everything. if you want to beat the thing as quickly as possible, you can do that in a few hours, but IMO, if you are playing that way, you are probably playing the wrong franchise to start with.
The biggest issue I'm having now with it is there there isn't enough to shoot at, but this is similar to FO:NV in that respect. It would be more interesting, for example, to have some random patrols, snipers in the cliffs, etc. that respawn after a day or two. There are plenty points of interest, and I think Obsidian did a better job of making them a bit more interesting this time around.
The arms and armor was especially well done, I think. I particularly enjoy the Tommy gun in a spray-n-pray-sort of way. The sounds of the .45 and the Thompson are especially well done, I think. There's nothing like emptying a drum into a giant sport plant with the gun sounding off and the brass clinking all over.
The story is still somewhat linear for me, but part of that is that i only want to do the quest in one way for ethical reasons. Your mileage may vary here.
Overall, I like this better than Dead Money, and I like the trend of more open gameplay. I'll give it a 9/10, and I would encourage Obsidian to keep up the good work. With a little more practice, they might actually regain a bit more of my respect in their ability to create a sandbox (if not sandbox-ish) game.