My first sentence is five lines long. It's not going well...

Well I suppose that depends on the disposition of the reader, doesn't it? I would guess that the majority of readers would agree with you, because the majority of readers are nigh on illiterate. However, if you get that one admissions administrator who enjoys sentence structure and the craftwork of writing (and not just the content), then perhaps it's going splendidly!

My own experience with graduate school was wonderful, although it wasn't similar to what one would have to do to obtain an MBA, as it was a full time, five year slog through a PhD in biophysics. If I had to do it all over again, I wouldn't change a thing.

I'm not sure I can give you any advice, given the qualitative difference in our goals. The one thing that I think is universal, however, is that it's best to think of what a fulfilling life might look like, and then to try and monetize that. If you love your job, and think that an MBA is going to allow you to further lead a life that you desire, then by all means it's the right choice (independent of financial cost). I suspect that, based on the fact that you're looking for advice in the first place, that you're not so passionate about debit and credits and personnel management and supply chain optimization and so forth (I'm falling asleep even writing these words!). In that case, it's certainly not a wrong choice, but should be carefully evaluated in simple actuarial terms, as has been suggested elsewhere here. But that's not so easy either, given that what happens in the average has little to do with what might or might not happen to the individual, thus rendering such calculations near worthless.