WM's Ultralite is quite a popular bag and will be fine for 3-seasons in most of the U.S. You may get a little too warm during the middle of summer (no different than any other 20 degree bag). I also doubt that you will get too many extremely cold nights through-out spring-summer-fall that will take you anywhere near the 20 degree rating, certainly nothing that a light/mid-weight top won't fix.