I have been writing about LED lighting since we started the ole blog over 2 years ago. For review, LED, is 5 to 7 times more efficient than incandescent, lasts 20 years and does not emit heat, which saves HVAC costs.
Now the problems:
Problem 1: They cannot seem to manufacture LED so the color is white.
That issue has been rectified, but only by a few brands, most notably Cree, Juno and Jesco.
Problem 2: For many of the undercabinet systems, you need a "driver" or transformer. You still actually need this in many low voltage applications, but electricians hate the extra wiring.
Have a look at this:
The new Juno undercabinet...white and driverless. Available middle of February 2009.