I've used other apps, including other 'slow-downers', and I really just couldn't find my footing in them. Some are horrid cross-platform apps with unfortunately chosen keystrokes, and pretty much all of them had terrible UIs.
After using them, I decided to write Capo. It was a need that, in my opinion, just wasn't being met.
I wrote an app that can run on your laptop, with your guitar on your lap, and you can control it very quickly and figure out a song with no fuss. I take guitar lessons weekly, and this app scratches my itch beautifully.
You don't have to visit deep menus, fancy settings panels, or anything out of the way to accomplish the main task which Capo does very well.
I hope this answers your question (albeit in a very wordy, roundabout way).
So the UI thing is quite important. Maybe even important enough to make me spend money on a second slow downer. ;)
Capo looks amazing, but if I'm going to drop consider dropping 40 bucks I want to compare :)
In Capo, you have all the functionality you need, with the limitation that after 5 minutes of audio pass through for a given song, audio quality will begin to degrade.
Learn a handful of songs with Capo, a handful with the others, and let me know (use the Feedback item in the Help menu) how it went!
ASD costs $30 and was a truly awesome app in its day. And it does seem to have all of Capo's functions, except for the nifty waveform display. But the UI is cluttered, overcomplicated, and ugly cross-platform stuff. If I didn't already own ASD I would just buy Capo. As one who does already own ASD I am still considering buying Capo.