Hi. I used the Clearblue Digital OPK and it worked really well for me. This type shows you a digital smiley face when you're about to ovulate. They're a bit pricey but I thought it was worth it. I got pregnant after four months, but this might have happened anyway. I actually found that by the fourth month I could read the tests without the digital reader anyway, so maybe next time I'll just go for the simpler cheaper Clearblue version. But for first timers I recommend the digital version which takes any guess work out. Good luck!
Cheers
Tilda