In my view MBAs are overrated. If you want a career as an employee, sitting on the board etc, then an MBA from a good University will definitely help.
On the other hand, if you want to be an entrepreneur - then having an MBA (especially, from a "generic" university) is more likely to be disadvantage than any thing else. I speak from personal experience. To be an entrepreneur, you need to be fluid, think on your feet - sometimes do "embarrassing" things that no "self respecting" MBA will even contemplate doing (I'll come back to this point later). Being a successful entrepreneur means coming back time after time after your product/service has been panned, living meagerly on bare rations, chasing your dream whilst everyone else around you thinks you're possibly mad. It means going out on a limb with no assurances, taking calculated risks and making things happen on a shoestring budget - all the while, fighting against your own inner insecurities and doubt. It means believing in yourself and your product/service even when no one else believes in it. It means doing whatever it takes to get your product/service known - if even it means banging on your neighbors door in a clown suit and offering samples of your product to them. None of these things that truly matter are taught in business school (or more specifically in an MBA).
Furthermore, due to the (unwarranted - in my view) high regard that people generally tend to have for MBAs, MBA graduates tend to have a sense of "entitlement". With that comes sometimes unrealistic high expectations (as though the MBA was a magic transformational pill), as well as the sense that certain tasks are "beneath one". An entrepreneur cannot afford such attitudes and does whatever is necessary to get results - if even it means running a marathon wearing a wedding dress, with the name of his company emblazoned across the dress - ask Richard Branson. On the subject of Branson (a billionaire), he once said - "Once somebody has a degree, they have something to lose".
Take a look at some of the top CEOs (who are employees), you will find the large majority have MBAs from some world recognized university.
Take a look at the FOUNDERS and OWNERS of successful organizations across the world and you will find people who have largely graduated from the "University of life".
MBAs do have their use though, they make fantastic managers. When my business grows big enough, I intend to hire them - but as managers or custodians - nothing more. The average MBA (employee for that matter) is "addicted to the paycheck" and falls to pieces when that supply is cut off. To be an entrepreneur, you need to be able to make it out on your own. Having an MBA does not help you do this.
Just to clarify, I hold an MBA degree (as well as a masters in a numerate discipline) so its not as if I have anything against post graduate studies per se - its just that in my experience, it has been more of a hindrance (or in 'MBA parlance' "it was not the most efficient allocation of my resources") - as it took me a very long time to rewire my brain to learn to use both my 'left brain' and 'right brain'.
To summarize (perhaps a bit bluntly), to be a successful entrepreneur, 'street smart' is more important than 'book smart' - to be a successful career man (aka "salary man") "book smart" helps a great deal.
So the answer very much depends on what you are striving to become - a "salary man" or an entrepreneur?
Remember (to paraphrase Bruce Lee): you cant learn Kung Fu from a book any more than you can learn how to swim from reading a book. Yes you'll learn the "body movements" etc - possibly even becoming an "expert" on different styles of swimming - but once you're actually in the water - its a totally different story all together - all bets are off.