I was working as a teacher full-time when I worked on my M.A. I took a few courses online thru the university I attended, though I also attended many in person. I truly LOVED the online classes!...not having to commute to campus, search for parking, drive in dark on the way home. I also liked that I could work on the class assignments any time of day. The interaction I had with the other class members and teacher were great as well.
As Perna says, you will have to be sure the institution offering the program is accredited, at least for your area of study. It's great if it can work for you!