I rented from DFBike in Siena. Here's the site

http://www.dfbike.it/

Very awesome bike shop, great selection of rentals and reasonable rates. I picked up the bike, they told me the price, sold me some clothes but didn't accept any cash as they would take it upon return. It turned out I didn't have a ride to return the bike so they came & got it and I paid then. The Wilier was one of the cheaper choices, they had better bikes available, though I can't recall the prices.