The Nitro 30 has been my go-to bag for about a year now. I love it, one of the few UL bags that is warmer than its given rating. I am a slightly warm sleeper though, but still, at 30F I sleep in just my boxers, and at 25F I had on a light baselayer, and was very warm.
Its not the lightest bag in the world, after I removed the pad holding straps, the bag comes in at about 28oz, but that is light enough for me. Great bag, and I would imagine the Nitro 15 is just as well made.