In my experience (I had three or four flats last summer while out running errands) it's a pain but not all that bad. I ride a hybrid with 38s though; maybe a road bike is different. If it's the front, the handling will feel a little funny and the wheel will sound funny; if it's the rear it will sound funny and feel a little different under you (that's the best way I can describe it). Then it's just a matter of pulling off to the side of the road or into a parking lot etc. and fixing it. Make sure you have a spare tube and patch kit, as well as tire levers. And don't forget to disconnect the brake before taking off the wheel, as well as reconnect it once the wheel is back on!!!