I normally don't hunt in rain, but I have. I've hunted in just about every condition known to the Greater Ohio Valley. Normally, if it looks like a heavy rain is coming, I'll try and hunt before it hits, and then I'm back out just before it starts to clear.
I've got a luxury box that lets me sit out of the elements and survey 2 1-acre pastures. Sometimes, if the rain has been really bad, a slight break will bring the deer out. I've nailed them within a half-hour of a toad strangler with horizontal rain.