Just recently discovered this. It;s like logic pro in a convenient performance ready package for only about £20. I had heard about it but it;s not included with Logic anymore (it used to be) and is a seperate purchase. So I thought I would check it out.
You can play audio files in there, even stems and set up fx sends etc. It's more for a a keyboard player than for live remix sequencing (ala ableton) but it depends how you perform. If you like to do effects sends from a track separate from the main stem and play synths live, but don;t need control over the structure of your track liek ableton, then this is a great option. If you don;t have logic then this gives you access to all of Logic's instruments for £20! Sculpture, Alchemy etc :-O And I do believe you can rewire mainstage into another sequencer.
If you do have logic and already know these instruments then you have the advantage that you already know the plug ins and can get cracking with it.
The only thing that stops me from using it live with spatialize is that you can;t play midi files alongside the audio stems. I like to send midi parts to different synths and tweak them and also have program chanegs for my synth happening at the correct part of the track. But I can maybe imagine this becoming a feature at some point though (the ability to line up a midi file next to the audio player plugin). But if you're playing keyboards in a band it;s perfect.
You can also route guitars and vocals into there for processing. Trent Raznor uses it live and changes his patches with a footpedal, though i;m not quite sure how it deals with latency? Perhaps on the latest MBPro's the processing is fast enough to crank up the buffer? Don't know.
I spent last night using the loopback plugin. Very well implemented and fairly intuitive live looper which i think i will use with my experiments in silence project to do live improvised soundscapes. Probably set up 5 or 6 on the bus at set level and use a korg nanocontrol to control the record, undo and fade out button. Yes it has a fade out button too and you can specify the number of seconds that it fades out over.
It;s well thought out for midi implementation too. The whole thing is bit like a massively expanded Reason Combinator in that you hook up your hardware controls to the smart controls and then assign the smart controls to synth parameters from there.
Also well though out with the concepts of Concert, set and patch level. For instance you can set up a smart control for the master volume in concert, a smart control for a plug in in the effects bus at set level. Then you can have multiple patches that you can recall throughout a track at patch level and have smart controls routed for each of these.
You can also configure the performance screen to show only what you want to see. For instance for a live looping set up I could see the waveforms of the looper scrolling, along with record and undo buttons. All configurable and midi assignable.
Worth a look at from a few different angles.