I’ve used them once (back in September to see if I would be ovulating when my husband was home on leave) and honestly, the one I used was annoying. The one I used had the test window and the control window. Both would come up with a pink line as you got close to ovulation. The test said was the line in the test window was the as dark or darker than the link in the control window it was positive and you would be expecting to ovulate within 24–48 hours. It also said to stop testing at that point because the next day it would still be dark, but the timeline would be different.
Also, just for kicks, I peed on one a few weeks after finding out I was pregnant to see if they really do come up as “positive” during pregnancy and indeed they do. The rest of what I have to say is a bit of a rant about them coming up as positive during pregnancy. So, a woman could have have a period while pregnant (not unheard of in the beginning of pregnancy), take one of these tests when she thinks she’ll be ovulating, have it test positive, think she will be ovulating, and instead she’ll have already been pregnant. This could cause her dates to be off when she finally gets the positive pregnancy test (until an ultrasound is done). Since some tests during pregnancy are only able to be done during certain times in the pregnancy, that could cause the pregnant woman to miss some testing.