I was actually testing this stuff out after seeing this article on ArmoramA and was going to do a little mini-review of my own based on some tests I did.
Got it at Michaels for $5 with the weekly 50% off coupon.
The tests I did look amazing and real. Wish I knew about it for my Gone Fishing diorama.
Reading the reviews it seems like this stuff might continue to corrode if not sealed. So my tests were more based on what to seal it with. These tests were done a few days ago on 1/2" squares of styrene. About 3 applications of the ruster was applied over the iron paint to achieve this level of rust. More apps = lighter rust. Also placing the parts in an air-tight container after application speeds the rusting as it allows the chemicals to react longer before drying.

All of these were handbrushed on (not airbrushed) and only 1 thin coat was applied.
A: Plain Future Floor Acrylic out of the bottle actually made it rust a little more and go a few shades lighter as far as rust color. Similar to Migs light rust pigment.
B: Prepared Matte Medium (from scenic express) This worked well. It darkened the rust slightly, but in other tests it left it the same and unaffected.
C: Ouch... Future Floor Acrylic + Tamiya Flat Base thinned with Tamiya Thinner. This reacted with the corrosive agent and frosted everything. The culprit based on other tests is the Tamiya Thinner mixed with the FFA. The Flat Base had no ill effects.
D: Decoart DuraClear acrylic varnish (Satin). Similar to Apple Barrel and Liquitex brands. This actually worked the best IMO.
E: This is the control with no sealant applied.
So beware what you use to seal it and always test on scrap first. Overall I love it and can't wait to apply it to a model.