I took depo-provera for an entire year (September 2007 - August 2008)and it has it's pluses and minuses.
I have two sons 17 months apart. After my first son, I took the mini pill as I nursed and it's a pill that is safe while nursing. Because I worked part-time evenings 3-4 nights a week, I had an irregular sleep schedule and couldn't remember to take the mini pill at the same time every day and hence ended up pregnant again when my son was 8 months old.
After my 2nd son was born, I wanted something where I wouldn't have to worry about remembering to take a pill everyday, but yet was safe to breastfeed. My doctor told me about Depo-Provera.
The nice thing about Depo-Provera was that it was safe to nurse and I only had to worry about seeing the doctor once every 3 months.
The downside, I was very irregular. I spotted all the time. A single pantyliner got me through each day, but I was still able to tell when it was my normal monthly because I'd bleed a little heavier 2-3 days a month. I was also pretty moody most of the year. I had my good days and my bad, but little things would get me upset more easy.
Another big downside to the Depo-Provera shot is most doctors will only allow you take it for 2 years your entire lifetime. A potential side effect is osteoperosis (I don't know how to spell that), so even though you don't have to take birth control daily, most doctors recommend that you take a calcium supplement twice daily.
Overall, I had the nuisance of the bleeding all the time, but it was great peace of mind that I could nurse and wouldn't get pregnant while my little one was under 1. When my youngest turned 1 last August, I switched to Loestrin 24 and am so happy with that. I stopped nursing and I know when I want to get pregnant, I just need to stop taking the pills.