I've been using a saliva microscope, Calista, thought it would be a better deal than conventional opks as it lasts for up to 2 years.
It's pretty good at showing when I'm close to ovulation, you're supposed to look for fern patterns - but I seem to get these for 4 days or so leading up to ovulation... since I've been checking other fertility signs it seems that I may ovulate as long as a couple of days after the last day I get ferns, so it's not an exact science. (apparently this is the problem with opks generally)
I've now learned that it's better to use opks in the afternoon, as this is when the hormones surge... so will try that this month.
Have also just bought some conventional opks to compare - got them from a website recommended on another thread here cost less than £10 for 20 opks and 5 pregnancy tests - arrived in about 3 days but not sure what kind of quality they are!
Good luck!