beachtent - OPK = ovulation predictor kit (I think). I've never used the sticks. The CBFM is admittedly an investment - the machine itself is not cheap and you do have to buy replacement sticks. But I love mine, because you set a window of testing when your cycle first starts each month, and then every day you turn it on during that window and it tells you whether you should test that day, or not. When it goes to low you stop testing. I wouldn't know which days I should be POAS or not with just the sticks!
In the first month it tries to learn your cycle and you use a lot of sticks (they come in packs of 20). After that, I think women generally use 10 sticks a month - last cycle I only used 6 - because the machine learns your cycles and adapts accordingly (incidentally, it's not meant to be suitable for women who have very long cycles).
The monitor tracks estrogen AND LH hormones - while cheapie sticks only track LH. Apparently, just before you get your LH surge, estrogen starts to build up, and the monitor picks this up and warns you that you are in 'high fertility' ... or basically, that LH is imminent and therefore possibly ovulation. This is normally a warning to me that CM will probably change soon to EWCM - I usually get ~2 highs before EWCM starts happening.
To put it bluntly (in TMI terms), if we start to SWI when we get highs, DH gets a chance to 'flush out his tubes' before the EWCM arrives and before LH surge happens.
When the LH surge is detected, the monitor goes to 'Peak' which is a warning that ovulation might happen 24-36 hours later.
I got pregnant twice last year using only the CBFM, and have only started charting this year. If you look at my charts you can see where I have recorded the monitor's readings against my temps. In most cases the CBFM gives good advanced warning.
Its not error free - but I think human error is the biggie. E.g. This cycle I forgot to check and missed out one test day and I think it screwed up the monitor's estrogen analysis because I didn't get any highs. Instead, we started to SWI when I saw EWCM from charting, but I feel a bit because I don't think we were as systematic and 'best chance' about it as previous cycles.
Even so, the CBFM still picked up the LH surge.