The frenulum is tremendously sensitive, and is one of the most sensitive parts of the penis, besides the glans. I'm circumcised, and am fortunate to have a tiny piece of frenulum left on the underside of my penis, and I can definitely say it is by far the most pleasurable part of my penis. With just one finger lightly rubbing the frenulum, I can reach incredible orgasm.
So my advice, make sure you keep your frenulum!