They vary a lot. Different widths, different curves, different amounts of drop.

Many years ago I trash picked a bike frame that was too small for me. I took it because it was clearly a really nice frame -- Columbus tubing, chromed lugs; made in Argentina. I built it up for my father. He didn't want a drop bar; so he picked up a Philippe brand mustache bar. Dunno where he got it, I'd never seen one before, and I've never seen another one. It was perfectly flat-- if you put it on a table, it touched the table at all points except the clamp area. Weinmann brake levers, the ones made for drop bars, out front; I think I put a stem shifter on the bike. Anyway, I used to borrow that bike now and then, raise the seat an inch or two, and... it was great. The combination of the mustache bar and the slightly too small frame made for a magical fit. It was a really zippy little bike, and I always wanted to set up another bike the same way.

Maybe ten years ago I got another mustache bar, but I was never able to get the setup where I wanted it. Probably didn't try it on the right bike.

Anyway, my takeaway is that it can be great... or not.