I'm a musician and while I think the idea is great, I think the mechanics in the demo are too limiting for most remixing work. Typically if I'm working on a remix I would change the tempo, change the song structure, etc. which doesn't really work well with the 'versions of the original stems' metaphor that is used in the application.

Then again it may be interesting to see how people get around those issues to get the results they want. Often times having limitations boosts creativity rather than hinder it.