+1 of the GooseFeet Gear down pants!
A while back I was torn between the MB & the WM (Flash) down pants), but I ended up going with the GFG down pants. What I got out of it was a pair of pants that was custom fit, and with my choice of materials. They also have 3.5 oz of down (which is 1.5 oz more than either the WM or the MB-they only have 2 oz), and at only 6.2 oz, lighter than the other options too! As well, I paid $160 for them, which is the same price (or actually lower) as either of the other pairs.
Now, these are with 10D inner/outer shell materials, so they are not as robust as the seat of the WM pants, however, I wear these to sleep in, so I don't need anything super robust. While wearing them around camp, I will just wear my DriDucks rain pants over them to help protect them.
Anyway, I highly recommend them. Here is a write up I did on them a while back:
GooseFeet Gear Custom Down Pants
Also, I disagree with Max about down pants being useless outside of the tent. Having these for standing around camp are a delight, and just as useful (for me) as using them with my sleeping gear. The same goes for my down parka. I never pull out my down parka (MB UL Down Inner) until I get to camp. I don't actually hike in any of my down items, save my BRG down hat, on occasion.
For those of us that don't actually need extra layers on our lower half to hike in, down pants are a great alternative to long underwear. They provide more warmth, and come at a lighter weight penalty. Of course though, this is just my opinion, based on my own personal experience for my neck of the woods... YMMV though...