I have been on twice. I got my first shot in Jan 2011, and had four shots so was covered until Jan 2012, except I went straight onto the mini pill. I was pregnant within six months, despite being on the mini pill!
I went back on depo in May 2013 as there was a short period when I really couldn't be pregnant (moving house, saving to buy a house, starting my career) and I needed something fullproof. I only had one shot, so it ran out roughly in August, and I have only been regular for the last two months. I am pretty sure that I am also ovulating as I am using a fertility monitor (as contraception) and it looks as though I am heading for ovulation as we speak!