Update! This location has CLOSED and only the East Washington store is open! Super Update! I heard that the East side location is closing too...
I bike everyday and everywhere so I'm pretty familiar with the Local Bike Shops(LBS) in Madison. Williamson on West Washington is my favorite for the simple reason that I have never had a bad experience there. New bike purchases, tune-ups, part purchases and repairs have all been good/great.
One of the reasons I like them so much is they treat you the same whether you are buying a $300 or $2000 bike unlike some other places. Unfortunately, the biking community has A LOT of snobs revolving around it so it's nice to have a shop where people act professional. Keep an eye out for their tent sales as they have a big selection with cheap prices.