This month and the month before I used opks and became a bit disillusioned. I had two different brands which were bought over the internet, so a lot cheaper than boots.
I used brand A the first month and only ever got a faint line at the time when my temperatures said I would have ovulated, the rest of the time nothing. I was testing at around 5pm. The line should have been darker than the control, so I didn't know if I hadn't ovulated or if the test was rubbish. We weren't ttc that month, it was just me being weird.
This month I used both brands at the same time, and usually tested at midday-ish to see if my LH was peaking earlier in the day.
With brand A I still only got a faint line at the time of ovulation but nothing else for the rest of the time, but with brand B I had a line all the time but at ovulation it went darker. So according to brand A I didn't ovulate this month, but I am now pregnant.
I think that these tests are really difficult to use and potentially really confusing, so I'd advise you to look at your other signs as well and don't rely solely on opks.
If you are interested, the brand B sticks I used were Accor from earlypregnancy.com - cheapies. (This whole exercise would have been far too expensive if I had gone for the expensive branded tests.)
A web site that you may have heard of, which I found put in in perspective was peeonastick.com - I think it is well worth a visit before you start using opks.