I love my cbfm as well!! I bought mine new off ebay too, with sticks and a pg test about same price i think, £70.
I never used opk only went by cm and what my (MyMonthlyCycles.com) predicted as ov days. The month i got pg i only had bd 2 mon and wed. MMC predicted i would ov 3 days later than cbfm showed me to, which was thursday.
I just got my first af after my mc and have already bought sticks for it, so im defenitely gonna use it again as i think its a great ttc aid!!! Its also really easy to use
Good Luck, hun! xxx