Don't get me wrong, I have used it. But it just seems cheap to me. The thieves guild quests, for example, you can fast travel, pick a lock, fast travel back, mission done in 5 minutes...
I know, i know, play how you want to....but this just seems like a cheap shortcut in every way. At least use the horse carrige and pay your 20 gold or whatever. I, for one, will not use fast travel again.