I am currently getting the shot, Depo Provera to be more specific.
The pros to this is cost, which is much lower (especially with prescription insurance) than pills. It is very effective, more than condoms of any sort. You get it once every 3 months, so no needing to remember to take it each night. You don't risk the same side effects as you would with pills such as blood clotting especially in the lungs. It is safe to mix with most medications as long as you consult with your doctor(s) about dosages and make sure they know you are on the shot (especially if you are taking any anti-depressants, as this will make them less effective)
As with ANY birth control shot or pill, or anything that changes your hormones or body chemistry to prevent pregnancy, you will need to use another form of birth control or preventative measures during the first couple of weeks when starting the birth control, and if you take any antibiotics you will need to use extra protection because this reduces the effectiveness of your birth control.
The cons are that you can gain weight, which I have not and have been on the shot first for 4 years, then I was on other methods, then back on it now for 8 months. No weight changes due to the shot. It CAN make you emotional or have moodswings, because it IS going to change your hormones around a bit. I just tend to cry more easily (and over silly things) that I wouldn't usually cry over, or even care about.
It's reversible, you don't have the "just given birth" after effect that you do with IUD's when they are removed/changed, and to me, it has been the best method.
I'm done sounding like a brochure on birth controls now