The first ingredient is water. Well, okay. The second is sodium caseinate. Sounds scary, but it's really just the protein that's in milk. After that comes some food-safe mumbo-jumbo that sounds spooky, but after looking up a couple of them, I figured that they sound worse than they are.
The interesting thing is the note on the can that because so little ends up on a loaf of bread, bakeries don't actually have to change their labeling if they use this on their bread. Interesting. So if you're buying bread from a bakery, they could use this stuff, and you'd never know.
If you're baking your own bread, you can decide if you want to use this or not. If it's not your thing, that's fine. Egg wash works.
On the other hand, if you're making one loaf at a time - or even two - you can end up wasting a lot of egg if you don't have another use for it besides the egg wash. That's where this product starts making a lot of sense. It sits in the cabinet and you spray a little on, and your seeds stick like crazy.
The one downside to this product is that not only does it make things stick to your bread, but the overspray sticks pretty well to your baking sheet. Using parchment paper under your bread will avoid that issue.
Also, some people on the King Arthur Flour site have complained about the nozzle getting clogged. Okay, I've got to say that this is something I appreciate about the company. They don't delete or hide the negative comments.
I had the clogging issue a couple times when I hadn't used the product for a while. To fix the problem, I've just pulled the spray thing off the can and rinsed in in hot water. That's worked for me so far. Turning the can upside-down and spraying until the product stops spraying is the proactive way to solve the problem, if you remember to do that.