I find some of the attitudes displayed on this thread a little disheartening, but I should probably have expected that. Go ask your attendings what you should do with the money and see what they say.
Save up for a down payment for a house. Fund your ROTH IRA accounts and your 401ks, up to max, even beyond any match. If you have kids, fund their educational accounts. If you have any other cash left, INVEST IT. You may still need a car to drive around in, fine. Go out and buy a safe 4 year old vehicle for 7-8k.
Doctors salaries' are headed down even while the real world (a place many of us have been insulated from) has become much, much, much more expensive. You do not want to be a 50 year old burned-out physician who is a few paychecks away from bankruptcy, who can't afford to reduce his 48 weeks/year q7 call schedule, and who has no hope to retire before 65-70 because they and their family lived above their means.
The habits you and your family will live with your entire lives, develop now. If you drive an M5 at 26, 27, 28, what is the chance that you will be happy or satisfied in a Honda when you are 40 and your stress level is twice what it is now. If you drive an M5 or similar car, then what kind of car do you think your spouse will feel entitled to? And once you become a parent then you need to carefully examine your consumption patterns, because those patterns will impact your children and their values. Ever seen My Super Sweet 16 on MTV? It's scary.
If you and your spouse will both work and you are fairly sure you want to live in a cheap midwestern city or town, then maybe you can blow your load on a new car. I still wouldn't advise it. Who knows what physicians will make in 10 or 20 years. If you want to live on the coasts or if you want your spouse to have the option to stay home with the kids, then save every penny, YOU WILL NEED IT. Let me share with you the finest piece of wisdom that you will ever read on these boards: You want to be in a position to retire as early as possible, because then you will truly have peace of mind, professional flexibility, and the opportunity to take risks with little downside.
I hope this advice helps at least a few people.
P.S.
This applies to rich and poor alike. Unless your family can write a check for a million bucks to buy you a nice house in a nice area that you want to set up a practice in, then say no to the new BMW, ask for it in cash and put the money to work for you. If you seriously want to use scholarship money to buy a car (like the dude on HPSP) then you need more of a financial reality check than I can provide. Oh and of course, to many people a hotshot physician in a sleek 911 turbo screams out not "success" but rather "sue me"... welcome to reality.