Leasing, financially, is almost always a suckers deal unless as indicated above you can deduct your lease. You cannot justify it financially w/o that deduction. Obviously there are other reasons to take the lease and typically a lease puts you in a vehicle you cannot afford to buy outright(the #1 reason buyers lease). Another top reason is drivers like to be in a new vehicle every three years and leasing puts that on auto pilot.
Leasing BMWs are typically less of a sucker deal than other brands due to the high residuals. But you will still be money behind with a lease vs buying. And the longer you hold on to a purchased vehicle the better the financials look vs leasing.
Don't let the dealer lead you around with the standard how much do you want to pay a month and how much do you have to put down BS. Do your research at BMWconfig, KBB, Edmunds and you should be able to go into the dealer, stand firm and get a fair deal.