I have seen a few variations of this idea and love the concept - unbolt your original wheel, replace it with one of these, and take off. No throttle or battery pack to mess with with and no wires to zip-tie all over your bike. Simple, elegant, and very cheap compared to almost any other form of motorized transport.
Factor in registration, insurance, fuel, maintenance, parking, and see how long it would take to recoup your price of entry. Or, just factor in the fuel and maintenence costs if you did 40% of your travel on this bike and see when your return on investment would kick in.
May you enjoy your commute today!