PHD created a bit of a buzz a while ago with their Minimus Down Trousers. Although attracted to the idea, I’ve held off buying a pair. At 230g, they are very light and no doubt very warm. While they add some flexibility in a sleep system, giving the option of taking a lighter sleeping bag (in my case a Western Mountaineering HighLite), they can’t really be used on the trail.
There are alternatives. One is the Integral Designs PLQ Insulated Pants. Using Primaloft has the advantage of being less vulnerable to damp, so they could be worn as trousers in cold conditions or under overtrousers. Another advantage is that Primaloft would give more insulation underneath your legs compared with down. They weigh a bit more than the Minimus trousers at 340g.
However, I already have alternatives in my gear cupboard. I have a pair of Lowe Alpine Powerstretch fleece trousers (230g), which are quite warm. They would resist compression even better than Primaloft and are quite good under windtrousers or overtrousers. If I used a pair of Montane Featherlight Pants to make them windproof and strictly comparable, it would add an extra 114g, for a total of 344g, not very different to the PLQs.
Another combination possibility is a pair of Lowe Alpine Dryflo Light Tights (130g) combined with a pair of Ortovox Merino 3/4 leggings (124g) which I bought for skiing. The total weight would be 244g plus 114g for the Featherlights, making 358g; not much different to the PLQs or the Powerstretch combo. The advantage of combining the Dryflos and the Ortovox is flexibility. I think I might give this combo a try in the Spring using my HighLite bag to see whether it works. If I use my Minimus Jacket (which I would take anyway) I would have not only quite a light sleeping system, but also a warm clothing system for camp as well as something warmer for my legs on the trail.