When it comes to realism, 1st ps is more 'real' than 3rd ps. It gives me feel that I'm the character I'm playing, wether its a shooter or RPG or even survival game(the one I'm working now, more like Penumbra I guess). I like the feel that I see trough the character's eye, more if I can see his/her body part like hands and feet(like FEAR series or Operation Flashpoint series)

But when we talk about fun, I'll choose 3rd person of course. Especially when the character we play is a female :p. And he/she has a whole load of move, good one I guess...

And I dont like games that give us option to switch between 1st person and 3rd person view.