just my experience on this subject,
I have a radar on a 6 meter Rib, with the dome mounted on the a-frame, that gives me about 5-6 miles coverage, depending on the height of the target of course.
Normaly i have the range set to 1.5 miles, because then I will be able to see the targets just infront of me, and the other stuff further out, i will think about them when I get closer.
In the dark or in the fog, the radar is the number one navigation equipment. But you have to remember to use the radar even when the sun is shining, and you are able the see further that the radar. Because if you can't read the screen to knoww what the little dot's are in a sunny day, you won't know in the fog either. So normaly my radar is started up together with my GPS as soon as I leave port. And I wouldn't miss it. When your out there the fog can come in on you pretty fast.
And if you are in the fog, you are normaly going to slow down a bit anyway, so you have time to read the screen, and it can be done even if the boat is jumping around.