I think VR would be fine for normal use if they just made the current in-game camera the VR view point. So you would still use you mouse/joystick to rotate the camera around your character, and looking straight forward would be as the camera is now. Then head tracking could let you still look around the field, but the game would play the same. A 3rd person viewpoint isn't as immersive, but it is still really cool.

I think it will make combat a lot more difficult. VR from 3rd person still cuts down on awareness, though not as much as being in 1st person. On top of the awareness drop, interfaces become more difficult to see and interact with, and you have to be very comfortable with your controls as you can no longer glance down at your hands to re-position them (I type without looking at the keyboard, but it is still incredibly difficult). I think it would be doable, and quite fun as long as you are comfortable enough with your controls.

On top of the increased difficulty on playing, if the game does not run at a very high and consistent framerate it will cause motion sickness very quickly. When I first got my Oculus Rift, I had a gtx 570, and with that I would be sick within 10 minutes of play in a game where you hardly moved. Once I upgraded my video card I haven't been motion sick since.