They're really not--at least for my purposes. And $25-50 for an episode requiring minimal cleanup versus spending 30-60 minutes going back through the audio and fixing things up is a no-brainer for me.

ADDED: Machine transcriptions are pretty good for a lot of things such as search and quickly skimming content. But if you want something that people can read as an alternative to listening to the podcast, you pretty much have to use human transcription or budget a bunch of time to fix up.