I've had a few cars with HUD and they were easily visible during the daytime ... I'll bet the rake of the windshield and the length of the dashboard DO play important roles in how it turns out, though! (and I wonder if HUD cars have a different windshield with better reflective properties? ...not sure...)
I've found some work a lot better than others.
I've driven a few Corvette's with HUD, and they work pretty well.
Lexus has to be the worst I've seen. A lot of them are really hard to read when you are have sun at a lot of angles.
I've seen a few in Bonneville's too, that were OK. Better than the Lexus, worse than the Vette. That actually might be a decent swap, as they are stand alone units.