I've tried two types recently. I bought one from Smarthome claimed to be as "bright as a 45 watt bulb" but uses about 6 watts. It looked exactly like a normal glass spotlight. I tried it. I'd guess maybe looks like a 30W bulb. It claimed to be "dimmable" but it onlt had two settings, on and off. It was wired in parallel with several incandescents. They dimmed it didn't.
Then I bought a high-efficency commercial can-type light from here:
http://www.permlight.com/
It claimed to be dimmable and the highest efficency efficiency made. Ran about $85 for one. It was bright, maybe about like a 60W bulb. Again, even when in parallel with incandescent bulbs it didn't dim.
I measured its power use and light. Both were very close to the amount of a normal compact fluorescent bulb but at 10X the price. And not dimmable despite the claims, although it worked fine on a dimmer (just didn't dim.)
So conclusion, if you want a dimmable light, I haven't seen it. The only advantage they have that I have seen over a compact fluorescent bulb is life. I'm replacing compact fluorescents once a year or two, these are claimed to have a 15-20 year life.
People assume that LEDs are an efficient light source, but usually they are no better than a normal fluorescent, sometimes less.