We've had similar discussions about this subject before. If you can get into one of the ultra-elites, it should be a no-brainer, just go. The two years that you spend is a pittance compared to the access you will have to professors, fellow students, and perhaps most importantly recruiters. Can't remember where I read this, but at no other time in your life will you have the opportunity to meet with dozens and dozens of firms from various industries.
If you can get into an elite school (check the b-school review forums for definitions of these) it's virtually a no-brainer. Unless you are in a career where you are already making a lot of money and have a well defined high-trajectory career path that doesn't require an advanced degree, the time and money spent getting an MBA will pay off quickly.
MBAs from schools in lower clusters are valuable as well, but require a little more thought an planning before you leap in. You will need to consider how much money you are making now and how much you will give up during school; whether an MBA from the schools you are considering will help you get ahead; whether said schools are strong in the region of the country you'd like to work; what placements are like, etc.
It's a no-brainer if you can get into the best schools, but an MBA can still be valuable if you carefully define what you'd like to get from the experience.