I've used it many times. It removes the rust an bluing quickly.
You may still have to do other work if the finish is badly rusted or pitted, as rust will roughen the surface, and a liquid is not going to fix that. But if its just bluing or light rust, you'll find that the stuff works fine.
Its the only Birchwood Casey "finish" materials that I find worthwhile.
As with all of this stuff, wear rubber gloves. Some of it is definitely unfriendly to skin and nervous systems.