This paper presents an immersive Virtual Reality (VR) therapy system for gait rehabilitation after neurological impairments, e.g., caused by accidents or strokes: The system targets increase of patients' motivation to perform the repeated exercise by providing stimulating virtual exercise environments with the final goal to increase therapy efficiency and effectiveness. Instead of simply working out on immobile stationary devices, the system allows them to walk through and explore a stimulating virtual world. Patients are immersed in the virtual environments using a Head-Mounted Display (HMD). Walking patterns are captured by motion sensors attached to the patients' feet to synchronize locomotion speed between the real and the virtual world. A user-centered design process evaluated usability, user experience, and feasibility to confirm the overall goals of the system before any sensitive clinical trials with impaired patients can start. Overall, the results demonstrated an encouraging user experience and acceptance while it did not induce any unwanted side-effects, e.g., nausea or cyber-sickness.