I used them for several months. I would start them on day 12-13 and get a lighter line and as the days progressed, it would get darker and darker until I got a + on day 15 or 16. My cycle has been goofy for the last 2 months and last month I used up the whole box to day 18 and nothing happened. It turns out that I Oed late (like day 21). Anyway, I've been charting my bbt this month and I like it a lot better. For one thing, it's A LOT cheaper. I bought a fertility monitor (I haven't used yet) because I'm tired of spending all that cash on the OPK's. Good luck to you!
The monitor has you do a bunch of test sticks (you pee on them) all month and it learns your cycle. The second month, it asks you to test a lot less and can tell you when your fertile days are. It even tells you to expect AF.
No mistakes. I would even ask DH which one he thought was darker (once they get pretty dark you are pretty much set).
Oh, I also have found that it's easier just to BD every other day from day 10 to 25.