I'm circumcised but, in terms of being on the receiving end of oral sex, I love nothing more than for my wife to lick, suck, nibble, graze her teeth or even gnaw on the area of my frenulum (pinching and rolling it firmly between her fingertips or scratching it with her fingernails works a treat, too). In fact, enough frenulum play can get me cumming with no further stimulus needed! However, most of the other guys I've seen her give blowjobs to tend to react badly when she gets her teeth involved in the oral action...
~Reese! :~)
P.S. ...have got a video of this favourite flavour of frenulum stimulus in my gallery, too, from which the following vidcaps were taken: