It's like the difference between a flourescent light and a regular light bulb. I own the SL 4AA and really like it. I get good white light for close up work, reminds me of those flourescent lights mechanics use for working on cars. The kind that has a hook that you hang on the hood or underneath.
Works extremely well for indoors. Take the 2D M*g and open it up for its widest flood setting. Then imagine if the light was white, even, and brighter!
I also have a Lightwave 2000 and 2 PT Attitudes, they have 4 and 3 leds respectively. The SL is brighter than them individually, and maybe collectively.
Do a search on these forums for the SL, there was an issue about them heating up but I think its been resolved.
When I need a light and size and pocketability is not an issue, I will grab my SL. Plus it stands on end for use in "candle" mode, to light up a room.
I like it but I'm sure there are those with the opposite view, always good to hear both sides before deciding. I have one of the older models I think and have never had any issues with it, but have also never run it for more than 10-15 minutes at a time.