We at ExerciseDesk.net recommend a mix of sitting in an ergonomic chair, standing, and active working through out the day. There have been a number of studies that show a small but noticeable decrease in productivity with actively exercising and working. Since these studies where done by new users of the equipment we expect the effects to reduce over time. These studies also showed that the reduction of productivity did not show up for every activity. Light exercise can be used during meetings, brainstorming, reading, and light typing work with little to no loss in productivity. However, if you need to do detailed mouse work or in-depth thinking it's best to stand or sit. When used moderately and intelligently exercising at your desk can give give you more energy and a productivity boost. Here are some of the studies that have been done:
This is a complete system that includes the treadmill and the desk. Reviewers give it high marks for it's ease of assembly, use, and durability.