I have one of those Topeak racks that cantilevers off of the seat post and has a quick-release mechanism, so takes about a minute to put on or take off. I bought it for my comfort bike (Giant Sedona), but was pleased to find that it also mounts easily on my new Sequoia. I usually just use a seat bag on the Sequoia, but this time of year where you want to dress in layers and then peel along the way (plus take some rain gear along), it is nice to have the rack with a trunk bag on it.
However, I don't think that I would use this on a bike with a carbon seat post!