I tried them before I read the book Taking Charge of Your Fertility.
At that time, they didn't help me because I didn't really understand my cycle all that well, so getting the timing just right wasn't very easy. They never gave me a overwhelming confirmation to convince me that I was ovulating.
If you have a pretty good idea, based on your body's signals, of when you are ovulating, they might be helpful, but I wouldn't depend on them entirely to tell when is the best time.
If you haven't read the above mentioned book, most of the women here will highly recommend it. I thought I certainly didn't need a book to figure this out, but after 8 months of TTC I finally read the book and started charting. I got pregnant on my first cycle. This forum is the reason I finally got the book and realized that my fertile period is very, very short, so timing was everything!
Just my experience! Good Luck,
Tina