I have an M.S. in criminology. I had an average GPA in undergrad and a stellar master's GPA. I think it counted as a good soft factor, but it does not do a lot to offset an undergrad GPA, since schools have to report that statistic to LSAC (and thus it factors into their US News ranking).
I applied to law school once before the master's and once after and had much better results the second time, but I'm fairly certain that my 7-point LSAT jump had a lot more to do with it.
One other thing the master's might help you with is making connections in academia. You might be able to befriend law professors if you're in a criminal justice program, and those professors might be able to help you get your foot in the door. I think this would be *most* effective if you waited to apply until after you completed your master's, though.