There's a local guy (JasonsDrivingForce - in the Raleigh community thread) that runs most of his races with a stroller. (He's down in the 19's for a 5k with the stroller, but don't let that intimidate you.) He's found he's often faster with the stroller because he can transfer some of his weight to it. Of course this doesn't apply to up-hill courses, which he admits can be very punishing.

I believe he posted a set of recommendations for stroller running, most of which I can't remember. However, one of the big ones was ALWAYS KEEP AT LEAST ON HAND ON THE STROLLER AT ALL TIMES DURING THE RUN. Sure, you may have that wrist strap hooked up, but do you really want to risk losing control? Even if you don't let it get away from you, it might turn suddenly and spill, and how would you feel then?

I add one minute per mile when I am running with my stroller on an average (i.e. not mountainous) course. In a crowded 5K, however, it could be more since you'll have to hang in the back or be very vocal and aggressive about passing people.