As far as rankings, I like the cluster system that Hjort has proposed; and actually, there is remarkable consistency in the rankings. Obviously, schools move up and down a few spots with each new ranking, but those moves are irrelevant. It takes a tremendous amount of time and money for a school to build the reputation and alumni base necessary to make meaningful moves in the rankings, no matter how you look at them.
Just look at Yale - impeccable name and a whole cr@pload of money, yet they can't move up no matter how hard they try. They don't have the long-term reputation, which means they have a more difficult time attracting top professors and top students, which then leads to fewer top jobs, which again makes it harder to attract top students and that professors will have more difficulty doing cutting edge research etc. etc. etc.
I would argue that, while rankings may change slightly every year, there is little real change in the overall position of each school over the short or medium term. 20 years from now, I bet is stays the same (barring some unforeseen geopolitical shift) for most top schools. Might Chicago or Kellogg gain a little ground and join H/W/S? Maybe. Might Michigan or Berkeley break into the ultra-elites? Maybe. Might one of them slip? Possibly. But really, I bet the class of 2029 will be shooting for the same top schools that we are.
Regarding lower ranked schools, I don't believe it's a no-brainer to get an MBA. As Rhyme pointed out, people with MBAs from lower ranked schools had no discernable increase in salary compared to those without an MBA. Throw in the cost of tuition, opportunity cost of 2 years, lost promotions & experience and so forth, and it's easy to see that if you aren't making 50-100% immediately after business school than before, it might be 20 years before you can recover the costs. Going to an ultra-elite is a no-brainer; going to an elite is almost a no-brainer but you need to work hard once you get there. Going to a lower ranked school - as I said each person must decide for themselves. Perhaps it will help you change careers or open up future promotions - but it sure makes sense to have a clear plan in advance.
I have a friend that went to a near-elite. 2 years and 100k in debt later and he's back at the same company (fortune 100) with a 20k increase in salary. If he had stayed and worked those 2 years, he probably would have earned the same increase in salary. He would have also banked 2 years of income and not incurred the 100k debt. Perhaps there's a greater chance he will be promoted now that he has an MBA. I don't know - and it will probably be a decade or two before he can be sure he's made the right decision.