I just received the Mighty Bright MiniFlex today, and have tried clipping it on my 3rd generation Kindle. It clips nicely on the top of the Kindle without the clip covering the screen. The grip is strong and does not move if the Kindle is jostled or tilted. (Note: the clip part casts a bit of shadow, which isn't big enough to cover any of the text on the screen. I figured it might be worth noting if it bothers people, though. I minimize this shadow by putting it at an angle on the top left corner of the Kindle, though the grip isn't as firm in this position.)
Since I haven't used it extensively yet, I can't comment on the strength of the bulb or anything like that, but the light covers the whole screen and the neck can be adjusted so that there is no glaring spot on the screen. So far, does the job!