I never build one. I was land-exchanged with the other day and landed on a land that had one, which was handy, and that's the first time I've ever had one. So, I'll use it if it's there, but I'm not that bothered about one. Solar panels and windmills are the way to go!
Never, the aggregate damage is a great deterrant on construction of a reactor. If I land on a tile that has one already built, it will be dismantled at the soonest opportunity. The only real safe place to own a reactor is out at sea with the monsters, and even that becomes an inviable option after a certain period of time.
And, yet, I eat my words. Active coal miners are in extremely short supply in the south these days, so i've had to resort to a reactor. So I guess I can't really say "never".