I've been using P Tracker (or Period Tracker) on my blackberry for the last couple years. I have only used the basic feature, allows you to enter your period start and end date, then predicts your ovulation and next cycle start date. It keeps a log of the data you enter and will calculate your average cycle. If you want you can add symptoms to the calendar as well as moods, weight and temperature. There's even a space for notes, and a toggle switch for recording if you have been intimate today. You can see what you've tracked using a calendar view or log view. You can even set it to alert you a certain time or number of days before your expected period, ovulation, or fertile window. If you want you can back up your information on your computer.
I have found this app to be quite intuitive to use and all I need in such a tracker. I use the free version, which means there are small but discrete ads at the bottom of my screen which I don't find to interfere at all with the app's usefulness (but one can buy the 'deluxe' version to get rid of the ads). My one complaint is that it is quite slow to load and sometimes slow to switch between data screens, sometimes to the point of my phone saying the app has stopped responding. t will respond if one waits long enough. That can be a bit frustrating but not the end of the world.
My favourite feature about P Tracker is that it is not obvious what it is - on the home screen it just displays as a flower called P Tracker - nothing about fertility or your period, which is good for privacy. Free is good too. I like free.
Now obviously most women (including me!) don't have a perfect exactly 28-day cycle, so the app is not going to be perfectly accurate, but it is close enough for me to give me a decent indication of when to start carrying around my products in my purse, or, I suppose, to remind me when to start peeing on sticks. I believe it needs to track three cycles before it can start making predictions for you.
I have not tried other, similar products that exist out there, and now that I have quite a long history invested into this app I doubt I will switch. But I am curious about anyone else's experience with other apps - what do you like and why?