This sort of thing is kind of controversial because some players will think it ruins the game while others will believe it to be the best thing ever. Still, I think in a modern first-person game where your body is "in the game" with you... it's an inevitable feature to have as an option.
Truthfully, I'm not generally a fan of third-person for computer RPGs. It's nice on the consoles with a game pad but for the mouselook+keyboard style, I prefer to "be in the head". However, some things are 1000x easier to debug from a third person perspective. I can easily test physics issues and see the actual points of collision. When animation is added, I'll be able to test the animation more easily, etc.. I even already found one bug where it turns out the avatar feet float about 1 centimeter off the ground. This wouldn't normally have been detected until network play was added back.
So, all that being said, there was an effect I hadn't really counted on. It seems to really add an extra sense of vastness to scenes. The following picture illustrates this well, I think. I could have taken it from exactly the same "camera spot" and I don't think it would have had the same affect. The sense of scale is really improved. Furthermore, because my camera is moving 3 meters behind the player's head, it adds a real sense of depth that isn't normally there.
So while it feels to me more immersive to be in the head... I think dramatically it may be appropriate to occasionally pop back out. At any rate, for now, F7 toggles this.
The world clipping is done pretty well now. I spent a few hours on it. And if you are backed up too close to a wall, it simply reverts to first person. I also think that for climbing ladders, I may resort to a tight-shot third person view even if you start out in first person.
The other thing I will add is a sense of environment. So if you are walking through cramped caves or dungeons, the camera distance will tighten up and stay there. 1) it's better at providing the sense of tightness, 2) it makes the camera bob in and out less due to world clipping.
...anyway, I'm pretty pleased with it and look forward to clamping down on a bunch of physics/collisions problems.