I agree with Colin, but I may be biased. It may clean the shelves ok but what about the rest of the oven. It can't take your door off and split it to clean properly, it won't take any internal panels and the fan off.
Probably ok for a basic diy job but can be dangerous if not treated carefully.
For what it costs it's well worth getting a professional in, after all you don't recommend windolene to your customers, do you.