For the last couple of months, i've been testing some led lamps with a single 3W (12V) led, and i'm surprised at the performance of these lamps.
I've dissected one lamp, and put some photos here:
http://www.mwlmf.net/gallery/3W_Diodelamp
The specs for the lamp are: 3W, 11-17V, 60 lumens, 36deg. spread, 3000k colortemp, 10k-30k lifetime.
It seems that the electronics inside the lamp limits current, because i can not see any difference in light output at 11V vs 17V. Also, they does not dim after a while, like standard diodes does with a little overvoltage.
I've tested several other led lamps with 18-21 leds in each lamps, and the usually dims considerably after some months. (Seems that they doesn't handle the charging voltage for my batteries...)
A side-by-side comparisation of these 3W lamps with a similar 20W halogenlamp, there is little difference in light output; the 20W are a little brighter, but not by much.
The downside is price...
Around $24 here in Norway...