I have three - the Mighty Bright Xtra Flex 2, the Kandle and the Octovo Solis.
The Mighty Bright's "high beam/low beam" design and flex arm offers the ability to adjust to one's preference. While the illumination is excellent, it is a pain to travel with; it is cumbersome and turns itself on too often.
The Kandle's design is very small, lightweight and fits/illuminates the Kindle2 perfectly. I am unsure how well it works on other devices, depending on thickness.
My current favorite is Octovo's Solis: http://www.octovo.com/product.aspx?cid=64&pid=22
It is extremely small, lightweight and elegant. Brushed aluminum casing and the LED is warmer (natural light-like) and not cool (bluish) like most lights. It uses a single AA battery and can fit in a pocket. The light turns on when the arm is moved to the 90° position for reading. One other plus - the light is focused on the device and does not spread to its surroundings which can disturb others. Unfortunately, it is designed for the Kindle 2 only.