If you expect LED lamps to dim to virtually zero brightness, then at present it doesn't happen. As the blurb in bhg481's answer states, LEDs need a constant current to work. In practise, this means they turn on at a set level. Lower than full brightness, but nowhere near the low level of halogen lights at near zero voltage. Go into any light store and ask them to dim an LED light for you. Then do the same for a halogen lamp. In my opinion, the phrase 'dimmable' LED being comparable to a 'dimmable' halogen lamp is a big con. Each lamp has their own benefits. I use both, but halogen less so.