I haven't done it since I was young, but the greatest part about it is hearing how the guy is enjoying it, feeling the cock harden/twitch as you do certain things, and then hearing him breathe quicker and moaning as the dick starts to uncontrollably twitch and spurt out that warm goo into your mouth, it's a weird feeling of power in a sense.
Having a hard cock in your mouth feels like when you put your finger in your mouth, but it's much bigger, warmer and it expands as you pleasure it. I also really enjoyed the feeling of not knowing when it would pop (like a pop-rock) and that made me all horny and shit.
I would tell the guy who I regularly gave bj's to not warn me about when he was cumming. I really enjoyed "learning" the signals for when it was happening, what stimulated it the most, what made him "tick" I guess.