I've been using the depo between kids since DD1 was about 11 months old.
For the first 3 months I had spotting almost the whole 3 months, but I figured it was worth it since I didn't fall pg, so I kept going with it.
I never had spotting like that again. I only got AF at the end of the 3 months, which was a good reminder to go get it again.
I do think the longer you are on it, the longer it takes to fall pg, but I really don't know. I did fall pg both times when I wanted to, so maybe I just wasn't trying hard enough, lol!
I have gained 8 - 10 kgs over the years, but I don't know if that's the depo, or the 2 extra kids & not exercising like I used to?

I love it, I swear by it. I get it in my arm too. Due for it now actually...