An MBA is what you make of it. But the most important element of it is to seize all the networking opportunities. Who you knows, and not the text books and thesis' you have written, will get you a whole lot further. If I had my life to live again, I'd have swotted harder at my GMAT and got into Harvard. But I was so traumatised by the mathematics that I wasn't about to get fussy about my GMAT scores. Now I know I could have done it, got a higher score, and gone to Harvard or one of the other top MBA schools.
But regardless of whether you get into one of the top twenty or not, just make the most of every one you meet along the way. An MBA course will throw you up against some very hot and keen minds. In later years you'll look back on how much you learned that wasn't in the syllabus.
I think the two greatest things to be gained from an MBA are the networking opportunites and the confidence you gain in your own abilities. I found a good deal of the papers were rubbish; academic bollix delivered to you from people that had ensconced themselves within the hallowed halls of academia and had never never ventured out into the business world themselves, however I learned so much from my classmates; we were able to share real world experiences and I was able to take numerous ideas away that I now uses in my own businesses. Does an MBA make you a better business person..hmm I'm not convinced that without the theory that I learnt that I would be doing anything different in my businesses from had I not done it' however what it do do is I did really well with my grades so it gave me a lot of confidence; I came out thinking I was not as stupid as I thought as I'm not a great academic so struggled with the GMAT but once getting onto the course was able to use my real world experience and was able to ace many of the papaers. The confidence allowed me to go on to start businesses in 3 continents.