I don't suffer from bad dark circles, but i do have a slight shadow and often get subtle bags from working odd hours.
In terms of coverage, this offers a good colour for light shadowing under the eyes, and manages to substantially brighten the eye area. I've heard from others that it might not be as effective for those with worse shadows, but for me, it worked brilliantly.
In terms of permanent brightness - i.e the reduction of dark circles when not wearing - it seems to reduce mine when i have them, but they never completely disappear, even when they are only light.
I roll this under my eyes, in a semi circle shape, then blend it out with my fingers. I find that the semi circle of product i apply, is enough to cover the area i need to. It comes out in a thin layer, super easy to blend out.
Mine is the fairest shade, which initially looks a little darker than my skin tone, but blends to a seamless finish. It feels smooth on the face and seems to last throughout the day.
I'm sure by now you've seen this in your local Boots, and Superdrug, it's around the £10 mark, which to me does seem a little overpriced, because basically, it's a glorified concealer. While there seems to be a hint of product to rectify your dark circles, it's not going to cure them forever. Plus, for those with bad circles, it's likely that there are better concealers out there, with better coverage for what you need.
If you've ever seen the film Thirteen then they put cold spoons under their eyes everyday to keep them from getting bags, I've always wondered if this worked... so maybe try that to perk up the eyes in a morning! Has anyone tried that solution?!