Denver is very good. I am able to make my 17 mile commute mostly on trail and path, there are only 2 miles of surface street and even those are light traffic areas. The city has a grid system of well established bike routes that can get you all around.

The main north/south path follows the South Platte River. From downtown to the south east there is a path that follows the Cherry Creek. Either one of these can get you to the downtown area and several other business districts along the way. There are several smaller paths and trails that feed into these main arteries.

Additionally, the public transit system here is very extensive and every bus has bike carriers. The major bus hubs and light rail stations all have bike lockers, making it very easy to do a multi-mode commute. They also have a program that will guarantee you a ride home in an emergency.

Overall, I think it is a great place to commute by bike.