Carbonfiberboy 

We tour on our steel, rim-braked tandem. Our all-up weight in sport trim is about 335. We run Avid 7 V-brakes with Travel Agents, using 9 or 10 speed Ultegra road brifters. This all works fine now. I say "now" because we did blow a few tires off rims on what were really minor descents compared to what one encounters on tour. The secret to rim brakes on a heavy bike is deep alu rims, 25-30mm deep. Once we switched to deep rims, no more blown off tires. That said, I would not want to make a steep switchback pass descent on this bike, even in sport trim. I would very definitely recommend switching to V-brakes and Travel Agents. That's quite inexpensive and the V-brakes are very powerful and easy to set up.

It's our touring bike, too. When we tour, we switch out the rear wheel to one with the coveted Arai drum brake, with which we can descend anything.

Yes, wet and dirty rim brakes in the rain, have to apply them at least 50' before anything really happens. That just means being conservative and also somewhat lucky. But luck finds those who are conservative anyway. Rubbing alcohol is wonderful for cleaning the rims before a ride.

My worst rim brake experience was doing a winter rando training ride, which all involve a lot of climbing. In this case, we climbed out of the rain and into very wet snow. The wet snow soon covered our rims with ice. Ice is very slick. It turns out that rim brakes can't generate enough drag to melt said ice. Luckily, we only had to come to a full stop a couple of times. The other feature of this snow is we fairly quickly had only one one usable cog in back, though we still had 3 rings in front. We descended down out of the snow without a major disaster, though we did get two flats, once on each end, while up there in the snow. Once down, we bailed along with another tandem couple, the course running about a mile from our house. We partied all afternoon and them drove back to the start in our truck with the other tandem, sorted vehicles and went back home. Isn't rando fun?