I've used heaps of ICs, I did a few recently because I have had a threatened miscarriage and I thought the tests might give me reassurance. I have since learned that this was a bad idea.
One day, the test was kind of dark but definitely fainter than the previous ones. Next time, it was soooo faint I could barely see it but then so was the control line. Then I did another one that wasn't even fmu and it was really, really dark. This was all with the same brand. I'm still waiting for an ultrasound to determine if I'm miscarrying but the hpt results don't help either way, they are all over the place.
The only way to really know is with a blood test.