I can't compare my cold weather experience to most of the posters here. North Carolina is relatively warm until you go above 4000' of elevation. With those disclaimers in place, I've spent a dozen or more nights in the mid to low 20's (°F) with pads and never been even slightly cool. Even two of the cheapest blue Wal-Mart pads (stacked perpendicularly) worked fine at 20°F. Were they as comfortable as an underquilt? Absolutely not. Did they compress and fit within my backpack as well? Of course not.
If you are going to routinely winter camp do yourself a favor and get an underquilt. If winter camping is an unusual event for you use inexpensive pads and have a great time.