We've used Parking Spot for years including back in Maryland more that 10 years ago. Response has always been favorable. Denver has always been great, with one exception where you may have been missing a driver or two. We use the advance booking and entry and exit are super easy. Weill continue to the use the parking spot.
I love that they direct you where to park and a bus is waiting for you. They drop you at your car and wait do you to start your engine to make sure your car starts in the cold. This happened to me - and they jumped my car on the spot. Very nice staff and reasonable rates compared to the airport.
You always have a place to park, and the shuttles run at decent intervals. All your shuttle drivers are friendly and helpful. The only thing I would suggest is have some covered parking.