First off, I cannon believe this is free. I'm making a donation today. Period.
Secondly, the ability to grab a loop and mute it before it plays back is awesome - I've done this before, the hard way in Live, but Sooper Looper makes it efficient and easier than ever. I don't like building sound on sound throughout a song, so the "grab for later" philosophy is fluid here.
Third, the routing integration with SL is great in Live. I love being able to throw loops into their own tracks and create clips this way on the fly.
Lastly, I've been using SL for about two weeks. The fact that I can bring it to stage so fast is because of the community here. I've only made a few posts, but have read many many more.
As an instrumentalist (Saxophonist) who cannot use their hands for controls, SL has changed my approach to improvisation with a computer. I feel like I've been a slave to pre-planned sequences until now. SL helps me keep the giant sound i've been maintaining, but without sacrificing the ability to create raw, off the cuff, from scratch. This, I have been sacrificing.