I'd never heard of them, and just have a regular motion sensor light aimed down into my run. I guess you can never have too much security though, so I'm subscribing to this post to read BYCer reviews...
I've had 3 of them for several months. I don't care what the nay-sayers have to say about "raccoons can't see red" and "why not just put any old blinking light out there" - I think they work!
I have two goldfish ponds right next to each other, almost joined, before I ever got chickens. I suffered terrible predation from raccoons - lost 7 Koi, about 5 big gold fish, and had multiple pond plants torn up on a nightly basis.
I bought 3 Nite Guard units (recommendation is for 4, one on each side of run) but my chicken run is REALLY close to my house and I figured that side was pretty well protected or at least less likely for the raccoons and possums to roam.
I'm telling you, not a single goldfish lost, no signs of raccoon predation or torn up pond plants. I think they're a good investment, although I wish each unit didn't cost quite so much. Oh well.
There's a UK version, different company, that sells "two red lights" solar powered anti-predator thingies. I'm going to look into those, too. I lost the link, though, so I have to do some searching.
I've used one in my garden for two years (just one even tho they recommend four---one in each direction). Never had a raccoon rip down corn (a first) or any other critter bother anything. In a side garden without it I pretty much lost every melon. Other thing I would add is that I have used it for two years and it is out there blinking away. Building my coop now and I plan at least two out there.