1) Excellent results. Read the label and CLEAN your stuff thoroughly of loose rust, grease, dirt, etc. before treatment.
2) Holy fudgesnacks Evapo-Rust is EXPENSIVE stuff.
3) It has a limited capacity. Once it goes dark, it can't hold any more rust.
4) Has anyone else perhaps mentioned that it's ridiculously expensive?
I love that it's water-soluble, it doesn't harm skin or polymers, it doesn't stink or create vapors, and it's safe to dump down the drain. Effects on other metals can vary; it seems to discolor brass for example. However, it's NOT an acid, so it generally doesn't harm other metals, and it does not continue to eat underlying steel or iron once the rust is absorbed.
But overall, pretty much the only downside is the eye-watering expense and the limited capacity.
Evapo-Rust is great for derusting small, delicate, high-value items. If you have something large, or it's fairly durable or not all that delicate, then a cheaper method might be better.