So for the heck of it, I bought a can of the Tamiya Metal Primer. I had no idea what it would be like, but gave it a shot. After getting home, I was worried. What color was the primer? The label didn't say. Turns out it goes on completely clear!
I've used a number of primers. The GW white was okay. I've used a black Ral Partha painted primer, which wasn't bad, but tricky to get smooth. Army Painter primer could produce good coverage, but needed to be shaken for a good while... and then shook some more to be certain. The best I've used up until the Tamiya was Testors, but the bottle was small. It produced good coverage while also smooth and did well being sprayed in varying humidity and temperatures. Only problem was that when the can ran low... well, bad results. I'm also leery of other primers like Krylon. I know some people claim they get good results, but I'd rather get something specifically for miniatures. Enter Tamiya.
Since the primer goes on clear, you don't get an added layer that could possibly cover details. I found that my behavior is better as well. I stick to the two to three light coats and don't worry so much about whether I got primer into that one nook or cranny. I've also found that my paints go on smoother and more consistently requiring less coats to get a solid color. Adherence of the paints is just as good as any other primer as well.
As to how it works chemically, I don't have a clue. All I know is that it does. And that without having a pigment/paint component, I don't have to worry about whether my primer coat is smooth or not. I recommend it to the curious or frustrated.