I can only say good things about getting the masters. First, it is a change and change is good, it stimulates the brain!
We are similar, I am a materials engineer that just went back to school for my masters as well. The company I work for has a liberal policy that lets you take a leave of absence to get your masters. It is good, except, you guessed it they didn't pay for it!
That being said, I think it is the best decision I've made yet. I have put in 6 years as an engineer and it was a great but demanding change of pace to go back to school. You can do it, put in alot of extra time studying upfront so you get used to that environment.
As far as salary, I have seen several places that value a masters as $10,000 per year more than a bachelors, but such surveys refer to the difference in first time job offers.
Both of us are probably in the same boat in that to get the great increase in pay we will have to take our degrees to a new company.
I like where I work at so right now that isn't my plan, but I'm not stupid the resumes have started to go out.
Best wishes, I highly recommend it.
EngineerDave