I have been using one since about the third month of TTC. The first three months we tried ovulation kits and then I read about the monitor and I thought it sounded easier than the O kits, albeit way more pricey.
I have found it very useful. I generally have very regular cycles (29 to 30 days long, occasionally longer if something stressful or I'm dieting but longest has been 34 days), and it has worked every month.
It actually helped me identify how my body feels and acts when I'm ovulating so now I'd probably be able to tell for sure without the monitor but I still use it. It's like a crutch. :)
It is very expensive however. I ignore their suggestion not to use sticks from two different packages because I think that's just a way to get you to spend more money and the sticks are expensive enough that I am not wasting any of them. It has never seemed to be a problem for me.
I tend to use six to eight sticks per month. The first cycle it will have you start testing early, like around CD 5 or 6 I think but it will then start to use your cycles to determine when to start you. I usually start around CD 7 and I O around CD 14 most months. (I also like a pp said, do not test the day after peak. Once I've peaked, I just ignore the monitor until AF arrives. Or hopefully soon, the day AF doesn't arrive! :) )