I tried one for a few months before I started clomid when I was TTC#2. I wasn't Oing at the time and my cy's length was very unpredictable so my results were very confusing. I didn't start using it again after I started clomid but I think I would have if I had taken it for more than 4 cy's but my pg cy was my 1st decent O cy on clomid.
If you get one I have a couple tips. If you can, get a scope that comes with a few slides (I don't think the little lipstick or compact come wil removeable slides) so you can keep a sample and compare from day to day so you can detect slight changes. Next I would suggest getting a scope w/ 100x's magnification. They are a little more tricky to come by but worth it. Most will have a 50X's magnification and will do the trick if that's all you can find. Anything lower than 50x's... Don't waste your time with it

Also, you can take a sample of CM and detect ferning that way
HTH!