I had two times my ride last weekend where I did this trading places thing. I passed these riders on a flat, then on the uphill they passed me, then I passed them on the way down and that was that. All fine, we just had our own paces on different grades. The annoying thing was later someone passed me at the very top of a hill at a fast clip, and then proceeded to go about 5mph slower than I would want to go on the downhill, but it was heavy traffic so I didn't feel like passing. Don't do that. I think he was trying to catch me for awhile and did a "whew" when he made it, forgetting what an annoyance he was going so slow right after passing.