I usually purchase gap, and probably will again - I understand the views from both sides.
But, firstly, you don't normally need to make a decision until the 6 month mark - you should be covered by full return to invoice on you regular insurance within that time period (check) - and gap insurance companies, including those badged as BMW, will allow to take out the gap policy before 6 mths at no additional premium (for the reason just stated), so you may as well save 6 mths.
Buying BMW gap is at least 50% more expensive than going to an indepenent but you do get the BMW backing whereas an indie financial house could go bust at anytime. There's an indie that does very good prices, has been round for 10 years, and seems to have good reviews, but I'd need to find and can't do so now.
There are two types of gap insurance - Return to Invoice and Replacement - Replacement is essentially covering you for inflation, as if you claimed on a rti after 5 years they'd be a shorfall on the price you'd need to pay. I've only seen RTI for m/bs?