Playing Sandstorm with no HUD is really fun. I love how it puts you in the moment and makes you rely a lot more on communication and map knowledge. But there's a big issue -- namely, you can't check how much ammo you have in your mags.
When holding R, you normally perform a mag check. This remains the same with no HUD, except you aren't shown what you have left.
A nice compromise could be a system that allows you to at least check your current magazine. Holding R could momentarily pull your magazine out and see if it was close to full, half full, et cetera.
Alternatively, NWI could make it so you can disable/enable individual HUD elements. That way you can keep certain parts but remove others completely.