I use the one step ones from amazon and I find them really useful - they dont tell you exactly when your ovulating but they give you a ballpark idea and I just find it takes the guesswork out of it, as soon as I start getting strong lines I know Ill get my positive in a few days so its time to get busy! My positive opk lines up with having very fertile fluid and little niggly pains so I know they are accurate for me, I dont think they are accurate for everyone though or ladies with pcos