A lot of both. First-person view affords precision to one's crosshairs for looting, shooting, and examining things close-up, while restricting your view to a tiny little FOV. It's more restrictive than immersive, because in real life I have peripheral vision that stretches to 180 degrees (while nowadays a 90 degree FOV is apparently considered excessive). 3rd person mode makes up for that, and I spend my time traveling in said mode. Fighting tends to be mixed between the two, first-person for shooting only... 3rd person for melee, dodging, taking cover, et al.
The scroll-wheel click is my friend in these games.
This is the same whether it's oblivion, skyrim (which I've only played on other people's computers... waiting on legendary edition for under $30), fallout 3 or new vegas.