I use the clearblue easy digital fertility monitor, but I've also used the generic CVS OPKs when I wanted to test twice a day (once in the AM, and once in the PM - 12 hours a part). The CVS OPKs were right on target - they generally turned positive within 12 hours of my CBEFM turning positive. So, IMHO, the CVS OPKs work as well as my $200 fertility monitor - you just don't get the fun of seeing a little "eggie" pop up every month

The only thing is, when you use them the first month, you can't really tell what a positive looks like. I remember I got some strong second lines a few nights in a row and I kept asking DH "do you think is it?" "really, do you think this is positive?" (highly annoying, I'm sure). Once I finally got the positive I could tell the difference, but it was pretty subtle up until that point. Good luck!!