Update April 17, 2015:

Not sure what other products are like, but I'd buy this one again in a heartbeat.
I have allergies, but I haven't smelled any odors with these products, but I've also been very careful.
I don't think that anyone should NOT buy this because of the warnings. They are there for their purpose and oxidizers ARE dangerous.
One thing I have found is that you need to put on a rather thin layer of the actual metal fluid.
Then just play with the application of the oxidizer. Make sure you use plenty of the latter though, and if you want a build up,
finish a layer and then place another. With a thick layer of the iron put down, the oxidizer has to work extra hard to make it all rust.
It's worth the lack of waiting to be patient and put down thin layer after thin layer. You can get some surprising effects with this.
And you really can't ruin anything by applying too much oxidizer. It's easier to use than you'd think, and quite forgiving.
READ UP ON APPLICATIONS BY REVIEWERS, as the packaging doesn't tell you anything.
Also, DO NOT USE THE SAME BRUSH for both bottles. Make sure you clean the brush for the oxidizer too, you don't want to
bring iron into the oxidizing fluid. Another great reason to be patient and put down thin layers. Cover completely with the oxidizer.

I've had success on cardboard flutes, (painted AND clearcoated no less), as well as metal and different model plastic pieces.

Original:
So far so good. I've read a lot of reviews about this one, AND others. Ultimately this arrived from Blick's Art Supplies, and I've had decent luck with them so far.

I was skeptical at first, but this is a real trial and error thing. I'm using this for modeling a diorama, and was pleasantly surprised
at how it turned a piece of cardboard into a genuinely rusted article. I haven't loaded up on the layers yet, but am planning on doing so soon.
I did a treatment on a plastic barn roof too, and with some practice and good buildup, it's going to look fantastic.

They have a lot of warnings on the box, I mean you ARE essentially breathing something that makes metal shavings rust almost before your eyes, so I'd be careful. But I didn't have the overwhelming fumes that I was concerned with. And as always, one should be gloved and have eye protection. The fear wasn't necessary, but more practice is. I think this is probably an excellent product. I see more of them all the time.

Be cautious and careful, but practice a lot, put on lots of the metal fluid (this bit has definitely changed since initial use, and I may have meant to say "oxidizer" instead), and vary your application time of the oxidizer, with repeated apps, and see how it works for you. Looking forward to practicing a lot more!

This stuff was so fun to use. I used it on a beach cruiser frame to give it an antique/ratbike look and it worked perfectly. I'd recommend finding a very small spray bottle and experiment with misting on the second coat stuff that causes the iron to rust (forget off hand what it was called). This gave a much more realistic/natural rust finish. Also what I found is that the iron coat finish when dried has a HUGE impact on what the texture of your rust will look like, so take the time to apply the iron paint as smoothly as possible or you WILL see brush strokes through your rust when it's finished. This stuff will "self level" somewhat but not completely smooth. In the end it looks/feels/behaves just like rust would, so fingerprints and oils will show up on the rust surface so keep that in mind. I also recommend painting your project a base color that you won't mind seeing if it should get scratched...my frame was already flat black, and after loading it onto a bike rack several times it has developed some scratches where the black has shown through. If I had thought of this before hand I would have painted the frame with a rust colored spray paint so the scratches were less noticeable. Also, don't be discouraged if you don't see the level of rust you want, it took several days for my frame to look the way it does and it seems to get better as time goes by.

Crafting

Perfect for reusing items, stuck right to the lexan scx24 body and wheels too.

Needed something to give a rust effect on a model I was painting. This was recommended by a fellow modeler and it worked great!