Right I have been and back yesterday. Used autoeurope.com who ended up renting from a crowd who had a prefab right outside the front door of Bergamo airport called Advantage. The autoeurope were way cheaper than Advantage own website.
Booked a Fiesta got a Fiat Punto (jaysus they are really a s*!t car). No problem with rental so far. Other car hire companies at the airport are about a mile down the road but there is a shuttle to them.
Took the advice here to buy the excess insurance from the website above. Saved 20euros alone on it from this one trip so Cheltenham, and other trips will be bigger savings throughout the year. Diesel and petrol a lot dearer than here (diesel 1.70-1.79).
Went into Milan and had to pay a toll of 1.80 for it, I hope that is the congestion charge. I like the Autostrate payment system, you pay when you get off it and it is relative to the miles/kms you used of it.