Unfortunately, this effect wears off by the time you graduate, which is when you need it the most.
Only if you're in the right graduate program. There are a lot of MS (and even some PhD) students in CS departments who are just looking to increase their base salary.
Furthermore, I've found that taking grad-school classes part-time is kind of an unfair advantage vs. being a full-time student. While I have time to really explore the material for the one or two classes I'm taking at once, while the full-time students are carrying a heavier course load, and TA'ing or teaching at least one more class, even if they're not working on their thesis or dissertation.
I guess I was in the right graduate program, because I think nearly all of my fellow students were there primarily for the intellectual stimulation and learning. I haven't heard anyone talking about increased earning power, and I certainly didn't expect to earn more with my degree.
Sometimes I thought it would probably be easier to be a full-time student, because they don't have to commute to work or schedule around meetings and other work obligations (including the occasional death march). I believe that it's hard work being a PhD student, but I'm not so sure the full time MSCS students have it any harder than the part-timers who have to balance a corporate job.
Personally, I'd love to get the chance to go back to school full-time, but there's the slight issue of never having earned my bachelor's degree to get through first. It doesn't seem to have had a negative effect on my performance in the grad school classes I've taken so far, but I don't think many institutions are interested in offering full graduate admission to undergrad drop-outs.
I don't think my MSc has made the slightest difference to my salary.