MAC Fix+ is a setting spray, makeup refresher, and mixing medium. It comes in a spray bottle with a lockable nozzle (brilliant!). It retails for $20 USD.
Personally, I use Fix+ mostly as a mixing medium for loose pigments. You just spray a bit on your brush, dip it in your pigment, and apply. It's really good for this. I also use it to take down any 'powdery' look after I put in setting powder. It's nice in the summer too. Store it in the refrigerator and mist when you need a cooling blast!
I don't really use this to set my makeup, so I can't really say how it works in that way.
Purchased by me.