I used them for four of the six months we were TTC. I didn't chart my temp but paid attention to my CM and had a pretty good idea of when I might be ovulating. I bought a bunch of them real cheap on ebay... like 40 for 15 bucks I think.
I used them along with charting and got my BFP 3 months after starting them. They work for some people but I know some of my friends never got a positive on them.
I used it Day 7 through 21 and they were all negative. Then my period was late. I think I conceived on Day 23. My sister used them and got pregnant the first month.
Re: OPK question