I've used em all - companies and vessels - and can concur we what's been said so far. However my cheapest and best experience was with Norfolk Line Dover - Dunky in both directions and on different ships. Only 12 places for bikes, each marked out in a large yellow box so no chance of one knocking against another. The crew had good tie downs and large poly bags to go over seat. They even loaded on and off first to avoid what is mainly commercial traffic. They ain't the poshest of ferries but have showers and a bar that is not overcrowed. I paid £39 once and £59 another time.
[Edited on 10-6-2004 by Athelstan]