I've conceived three times using the CB fertility monitor. Two were MC at around six weeks but the third is now my amazing three year old daughter! So, yes it does absolutely work. Oh yes I was 42 when she was conceived.
Here's what I know:
The OPK sticks you usually buy only track your LH surge. It is possible for your most fertile day to happen before the sticks pick up the LH surge - in which case you are unlikely to conceive despite what the OPK says.
Where the CB monitor really helps is that it tracks the rise in Estrogen which catalyses the LH surge so it monitors both. Your first cycle is a learning curve for the monitor-in fact it keeps learning as it stores up to six months data.
Basically though it will give you a number of days before your LH surge where your estrogen is rising and you are highly fertile. It should then pick up what it calls your peak day of fertility ie the LH surge - this might sometimes be over two days depending upon how much hormone is produced. After the LH surge/peak day(s) it will also give you a high reading before dropping down to low.
I have been doing a lot of research lately and it would seem that the days you are actually most likely to conceive are 1-2 days BEFORE your LH surge. But you can also increase your chances of conception by trying every other day during your fertile period (which seems to last about 5 days in all). The research also suggests that if you for some reason miss the high fertility run up to the LH surge, say you are ill, and also miss the day of the surge then your chances of conception on say the day after the surge are very slim.
So the research suggests you need to focus on the five days before your LH surge and ensure that you hit either one or two days before the surge is detected.
If this is the case then the CB monitor can indeed help as it will identify the five or so days of high fertility leading up to the LH surge. Ifyour cycle changes month to month slightly it can detect this and make sure you still get to identify the days prior to the surge where you are more likely to conceive. OPK's can't do that.
The research suggests that just relying on the day of ovulation to TTC significantly reduces the likelihood of conception in any one cycle so the average OPK alone isn't going to help much (unless you are charting alongside and know how to use the charts and OPK's together effectively-this takes time).
One thing to bear in mind though is that as you get older the amount of LH produced falls. The CB monitor checks for LH in the morning -but in fact in most women their highest levels of LH are picked up in the middle of the day/early afternoon. For older women the CB monitor might therefore not detect enough LH in the morning and therefore never give you a peak reading - just lots of high readings. I don't know your age but in this case you would likely be better off with charting and OPK's together.
In summary:
As you have got the monitor on loan and you only need to buy the sticks it's definitely worth a try - it worked for me three times and I'm ancient! If you find you are just getting lots of high readings for more than two months then you could swap to charting and OPK's...hope this helps....