So, make a guess on the efficiency of the regen process. There are losses in charging the battery, small losses on discharging, and losses associated with using the regened power. 
You will always be better off if you drive in such a manner to eliminate any braking. Slow acceleration and slow speeds use less juice than going fast as quickly as possible. Maintaining a slow consistent speed will result in better fuel economy, whether in town or on the open road. Energy consumed by pushing any object through the air will increase with the cube of the speed.