My old Colnago Super has carried a rack for most of its life. it is an old Blackburn Aluminum rack, bolted to the triangles of the Campy-Style dropouts, p-clips to the seat stays.

Many of the newer racks have a center strip (mud, tire protection) which would be nice to have, although I had put some plastic on top of mine for a while.

Anyway, I don't notice the rack when it is on the bike empty... at least I don't think so. Somewhere around 20 or 30 pounds, the rear-end of the bike starts feeling a little flexy. I'm not quite sure what causes the feeling, but the bike still is ridable, just I try to be a little more gentle with it.

I built a pretty heavy duty rack for my Cargo bike. I've had a few 100+ lb loads on the rack. The trick is not allowing the bike to tip over.