Had an inspection one on mine recently. Got a specialist to do it and was charged £405.
You have to offset what you'd save by using a specialist against a possible lower resale value if you decide to sell the car and it doesn't have a full BMWSH. Continuity of a BMW warranty might also be an issue to consider. Many enthusiasts recognise that a full BMWSH does not necessarily mean that the car has been well looked after. It should do, but it does't always. It depends how good the dealer is and how many M cars they see. Likewise people recognise that a good specialist who regularly services M cars is probably as good, if not better, than a main dealer.
What sort of mileage does the car have and how long do you plan on keeping it? If it's high miles and you plan on keeping it I'd suggest a good specialist. Use the money you save to treat the car to a few goodies.
Edited by Northernman