If you can swing the time and the money, go for full-time graduate school. I've done it both ways, and as intense and crazy as the full-time schedule seems, I found it much harder to go part-time.
The very best option is to attend grad school directly after completing an undergraduate degree (for any of you student members reading this). Although you'll think you want a break after 16 straight years of academia, there'll never be another time when you are so tuned into good study habits. Once you start working, you'll enter another stage of life, and you'll either want to revel in having nothing to do after a long day of work, or you'll want to finally try that dancing/karate/scuba diving/cooking/church group that you never had time for before - and all that spare time will fill up. (Not to mention if you want to date, get married, travel, or raise a family.)
Being immersed in full-time study also gives you more opportunities to converse with fellow students and faculty, because you'll be much more apt to hang around your department or a research laboratory. Many departments host informal lunch topics, where you'll hear about the range of research going on, or you may be asked to give a short presentation yourself (great experience with a friendly audience). Although you can certainly participate in study groups as a part-timer, it's still not the same as hanging out in a group 'til 2AM, sweating the details of a homework assignment or quizzing each other for the next exam. You'll also have more chances to drop in on a professor and shoot the breeze and to use whatever special equipment or library facilities are necessary for your work.
Financially, try for a research or teaching assistantship.
It will probably turn your one-year master's into two (for engineering),
or lengthen a two-year MBA program even more, but you gain on two fronts.
First, you can get decent money and usually some kind of tuition waiver
(e.g., out-of-state becomes in-state). It pays to apply to a number of
different grad schools to see who will offer you the most (assuming you
have the option to move), as school support can vary enormously. Second,
you gain tremendous experience. In a laboratory environment, you'll be
working with older grad students, post docs, or the professors themselves,
and probably be on the cutting edge of technology. If you become a TA,
it'll be a scary but extremely satisfying experience to share introductory
material with undergrads. As they say, you never learn something so well
as when you have to teach it yourself! And both research and teaching assistantships
look great on a resume.
NO! Keep working and do it at night....
Wouldn't we all love to go back to school? Maybe and maybe not.... Waking up at 7:00AM, going to class, coming home at lunch to study, working in the lab, coming home to study, meeting research partners at the library, coming home to study. Wait, when do I eat?
I have to admit - school was fun, and work is fun too, but I'm addicted to my paycheck. The thought of student loans and low paying jobs is not a road I want to travel again. I don't have a husband to support me, and I know mom and dad aren't going to help out this time. But my company will! How can I turn that down?
An "all expenses paid" trip to a local University - in 4 more years I can have degree number two and all it cost me was BOOKS. I'll still have my job that I enjoy very much, I'll have 4 more years of work experience on my résumé as well as my second degree. I still get to be a part of the professional world while I make new contacts in the academic world. And I can stick with the standard of living I've become accustomed to.
In a perfect world, I'd be able to afford going back to school full-time. But with mortgage payments, car payments and the rest of my bills, how can I afford it? And why would I want to when my company will pay for it for me! Granted, I will have no free time for the next 4 years of my life, but I will come out on the other end with work experience, a new degree and still be debt free.