NJW and JCO have good information. As a financial planner/CPA here is my take on lease vs. cash purchase vs. financing. If you cannot afford to pay cash and so must pay over time then you have to choose between financing and leasing. If you lease then your payments are lower because you do not build equity (and also do not pay sales tax except on the lease payment amounts). Leasing may be the only way that some may be able to afford the car. Leasing is also advantageous if your company is paying the tab - generally you can write off more of a lease expense than the depreciation allowed. But, remember you won't own the car at the end of the lease. This is good if you have beaten the car up or want a new one. One more point on leasing - it is costly to get out early. If you might want to trade up to a new car in, say, 2 years when you have a 3 year lease, you will likely have a pretty big loss to cover. There is also a problem returning a leased car with modifications, bad dings, worn tires, excess miles and the like.
If you finance a car the interest is not deductable unless you take out a second mortgage on your home or use a margin loan. Both of these options are dicey because it puts your house at risk (sort of) and margin interest is only (supposed to be) deductable if used to finance investments and then only up to the amount of your investment income (it gets technical - see your tax guy). Making the interest deductible is very important in bringing down the after tax cost of a beast, and there may be ways.
JCO is right - to convert the money factor of a lease to interest rate you multiply by 2400. If neither the lease nor the interest expense are deductable for you then it largely comes down to the cost of money.
If you think that the beast will hold its value then I think that the constraints of a lease may be a problem. A quick trade to another model or year would be more difficult than if you had a loan or paid cash.
Since lots of people have the ability to use any of the methods available, I have done calculations for clients on this issue many times. For a person with very good credit, assuming that the car is for personal use and that interest will not be deductable, it all comes out pretty much the same as to the cost of money. If BMW gives you 9.35% as a loan rate or as a lease rate there is not much difference if cash flow is not an issue. If cash flow is an issue, then you better lease. If you pay cash and save the 9.35%, well that is a pretty good return on your money (at lease given the market since March 2000, before that you wanted 25% per year, huh?)
One last thing: there is a BMW program to lower your lease money factor (interest rate). For each additional month's payment deposit, up to 8 or 9, they will lower the money factor by a little bit. The deposit is refundable at lease end. I have calculated in the past that if you have the money to put down it is advantageous to pay the maximum deposit. As I recall the rate could be lowered to about 4% this way. Even calculating the lost earnings on the deposits the deal was much sweeter. Be sure to look into this if you are leasing. And do the math! It pays.