Mine was 4 nights per week for 2.5 years. Not all programs are during the day and force you to give up working. However, working and going to school left me time for nothing else. I had to reintroduce myself to my old friends after graduation.
I have an MBA and I am a software developer. Looking at it purely from a financial perspective, I wouldn't do it again unless: a) I could get some tuition assistance from my current employer and b) I could identify specifically how it would help my career after graduation.
From a knowledge perspective, my emphasis was in Finance and I learned a lot about corporate valuation, capital structure, raising capital, foreign currency, exchange rates, arbitrage, etc. Most of all of that was theory. If I were to get a finance job, I'd have a good basis for all of the things I'd still have to learn to apply those theories to reality. Other than my emphasis classes, there were some really great classes (Operations, Strategy, Ethics, Entrepreneurial studies, Negotiation and Mediation), some "ok" classes (Accounting, Cost/price, some applied math classes) and some classes that were completely worthless (touchy feely HR classes, classes that taught the management theory de jour, etc). I read more business books than I can count, read and discussed many, many case studies, wrote dozens of papers and gave countless power point presentations. This was useful in that I was exposed to a lot of opposing ideas, learned to write for business, learned how bad power point is at conveying information and generally got better at public presentation.
Generally speaking, the value wasn't in the coursework itself, you can get that for almost nothing by reading a bunch of books. There are lots of online classes that you can take for free and you can go to toastmasters to improve your presentation skills. The value was in the other people in the program: a) class discussion is between students who all have work experience and come from different backgrounds and b) networking.
Not all MBA programs are created equally, investigate the curriculum and talk to some current students. Find out how many classes are taught by faculty vs. adjunct professors. Adjunct professors aren't always bad, especially if they are currently working in the industry they are teaching classes about and are willing to help you with networking after graduation. Find out what style of program is it. Mine was more entrepreneurial focused and less "big management" focused, so it largely avoided the issue of cranking out a bunch of group think clones that think they can solve any issue by applying some classroom theories. With all of the reading, writing and presenting, I can see how easily it would be to fall into that trap after you've cleverly navigated so many various case studies by applying various popular management strategies.
If you want to start a business, some MBA programs can be helpful because of the networking and because you'll have enough knowledge of various business topics to better engage accountants, lawyers and other professionals. If you have a corporate job, it can be a good tool to help climb the management ladder - some companies require it to rise to certain levels in management. If this is true, see if they'll pay for part of it.
I personally think all of the programs are overpriced. I also think the actual value of the MBA has decreased as more universities crank out more MBAs semester after semester. If you can two or three immediate benefits, if you can find a style of program that meets your needs and if you can afford it, go for it. You won't be worse off coming out of the program, but you might not be as better off as you hoped to be.