Yes, I've got fenders. Fun story: 30 miles from the end of a rainy century, a guy at a rest stop offered me $100 for my fenders. I should have taken him up on it -- they only cost $35 and it didn't rain the rest of the way.
I just eyeball the clearance, maybe 1/4-1/2"?
Two things to consider. What's the largest tire you'll run on a given bike? -- make sure you've got room for that under the fender. And where are you going to ride? I've very careful about getting off clean, paved roads. You don't want to get a large piece of gravel or a stick jammed at the fender, even if it's got a quick release it's jarring. I'm not sure what I'd do on a brevet with long gravel stretches.