Several months back I recruited three members of our crack review squad to do some head-to-head reviews of the three most popular "slow downer" programs out there: Amazing Slow Downer, Song Surgeon, and Transcribe!
I've included links to the complete reviews from each member below. In reading the reviews several themes emerge: Amazing Slow Downer has long been popular with banjo players for its straightforward interface and good sound quality, and this continues to be the case. Song Surgeon Pro hasn't been around as long, but has the most functionality of any of these tools. Transcribe! has also been around quite a while, but it is more targeted at people wanting to transcribe audio files to sheet music or tablature.
Instead of trying to summarize everyone's reviews, I'll just link to them here so you can read them yourself!
'Hey Jonty....!' 22 min