My husband and I purchased this monitor just before we got married, and have used it (against its recommendation) to prevent pregnancy, as well as to get pregnant. We have had great success, so far- we were able to keep from concieving when we were not wanting to, and the first time we tried to get pregnant, we did. Our son is now 7 mo!
The monitor will tell you when your fertility signs are "low" "high" and "peak". If you want to concieve, having intercourse on the high and peak days will likely accomplish this. For preventing, you have to be a little more conservative- we usually used condoms during high days and most often abstained during peak days. But after your peak, you can be pretty confident that you will not concieve.
The only part that we consider a bummer is that you use a box of 30 test sticks over 3 months (about 10 per month) and then you have to buy more. Sticks average about $1 per stick, so it does add up. But, its less expensive than a baby! We have had ours for 2 1/2 years, with several months off from using it during pregnancy and postpartum, but a friend of mine has been using hers for at least 6 years, and have been sucessful in preventing pregnancy. Several other friends have also been successful in timing conception and pregnancy using the monitor.