Do you get used to having no crosshair? I would like to try a no HUD game at some point. (currently in new vegas)

Yes, it is doable, and actually quite enjoyable except for: no iron sights for some weapons. No problem when you have tracer rounds in a minigun, but a hunting rifle you tend to find the spot on your TV screen, rather than on the weapon.
The other thing is that it is hard to know if you are stealing or not. I mean, it's right there, but I don't read it every time.
One alternative that I do is drop the HUD to about 4 on that scale. I also have mine in Light Blue, as it is easy on the eyes in dark spaces. By changing to 4, the HUD will nearly disappear in daylight.

I never had the guts to try it in Fallout 3. In New Vegas you just look down the sights of you weapon so you don't need the HUD or the Crosshair.

Have begun my no HUD game, playing on hard, and not using megaton (avoiding all towns and convieniences) Am going for full survivalist, last man alive kinda stuff. No more than 5 stimpacks at a time, no more than 3 weapons, carry only a realistic carry amount.

That seems very difficult so good luck. Especially since on hard even a missile doesnt even drain half a super mutants life at point blank. If the sniper rifle or something could actually snipe effectively on hard then I could see it being possible but I havent found a one hit head shot kill on hard yet.