One of the things about Fallout 3 and New Vegas is that I never seem to use the Fat man (Except for outside the GNR buliding on Fallout 3 but that is the only time I ever use it).
I don't like using it. While it has a big explosion and it deals a lot of damaage, I just don't think it's fun to use, and blowing someone up with a nuke ins't satisifying enough.
I prefer to use normal guns, as they are fun to use and I can cripple an enemy's limbs easier adding to their suffering, before I put a bullet in thier knee err I mean brain. Shooting an enemy's arm or legs off is quite satisfying.
I have recently started using the Meltdown perk, which is also quite good, but not for crippiling limbs :'(
I am a legendary weapone whore, I like finding them and using them. Not just on Fallout 3 & New vegas, but on other games such as Skyrim and Fable 2 & 3.