Usually, I talk as if I were actually my Player character. But we are playing in Japanese language. And our language tend to omit subjects. So I rarely actually use "I".
When we play we are always speaking in character, or desribing our characters actions unless we specifically say that we are speaking out of character.
Personaly I prefer to use I, since when I'm roleplaying I am that character, not somebody narating what some character does. As a DM, I also have my NPC's talk using I, but I'll make very clear who it is talking by adopting certain poses or accents and by talking arround a conversation. But the coversation proper is allways I