I'm riding Specialized's Phenom saddles, chromoly railed on my mountain bike and carbon railed on my road bike. I first tried this saddle out on my earlier mountain bike and found it to be super comfortable. Before I even had a mountain bike this year I was riding a Brooks titanium Swallow on my road bike, which is really nice, but I wanted to have a consistent setup across two very different bikes. So, if you're interested, I have a highly coveted saddle for sale.
Before I was on that Brooks Swallow I rode an older Selle Italia Flight for years. I will never go back. While extremely light, the Selle saddle was way too narrow and uncomfortable on rides longer than an hour, road or mountain. I also rode on an older WTB saddle, the one with a wide rear and pronounced curved nose. That was really comfortable, but not as light or as maneuverable.
The Phenom was engineered in Specialized's Body Geometry lab to relieve problems of soreness and numbness, hence the center cutout and width options. The carbon railed Phenom Pro also takes the edge off of road bumps while lightening my bike a tad. The overall shape provides just the right amount of support without digging into my legs, as well as the ability to slide off the back in really steep and technical descents.