I think it's rubbish, personally. At least, it doesn't seem to be working for me. I get full ferning on days that I know I'm not O'ing, and nada when I think I am! This month I used both MB and OPK to see if there was a difference, and got a positive with the OPK on day 18, around about when I'd expect it, and nothing from MB! I'm really disappointed in it, cos it sounds so good, and it's pretty expensive...
I had a similar experience with MB Erin. I used it before I tried to get pg with DS2, and in the end, I gave up on it and went back to OPK's (internet cheapies). I found OPK's to be FAR more accurate for me than the MB ever was. I found the MB just confused me, and I was a bit disappointed really considering how much I paid for it. I know I've probably spent far more on OPK's than I would've if I'd stuck with the MB, but if they give me a better result I'm happy to do that.

It seems other people have a lot of success with MB, so maybe it was just me