It's the combination of light weight, small pack size, excellent construction and reasonable cost make it a great value choice for 3 season backpacking. The Lithium is a mummy style sleeping bag featuring 800+ fill power hydrophobic down, a full length zipper, lots of loft and an extremely effective hood design.
Marmot says:
Tally up the ultra light fabric, high quality down and a minimalist design, and the Lithium stacks up to be our lightest bag for mountaineers and backpackers. Good in conditions down to zero degrees Fahrenheit, the updated design features on this bag help reduce shifting down and increase ventilation.
Part of Marmot's ultralight range of sleeping bags, the Lithium offers a superb balance of quality, warmth, low weight, packability, and performance versus cost. It's comparable to many more expensive products, as it is lighter and more compressible than almost all of its competitors.
The Marmot Lithium uses 800+ fill power down protected with Marmot's Down Defender to help the down to stay dry longer, loft better when damp and dry more quickly than untreated down. The Pertex Microlight fabric offers a combination of lightweight and good durability plus it's DWR coating will offer some water resistance. The fabric feels really soft which allows the down to loft freely and the slightly brushed feel of the Microlight lining fabric is extremely comfortable against your skin.