Pot helps my back pain moderately when it's flaring up. You have to remember to still do your exercises, or it can make you seize up more after a few days.
Inactivity is bad for your back.
Now, for my neck pain, which is way more chronic, it's FM (fucking magic). A couple hits and the pain just flows out of me and I'm loose enough to easily do stretches effectively which normally elude me. If I don't stretch, it doesn't get worse, it just comes back more quickly when I stop smoking.
So, YMMV as to back pain and types of pain. Pot works better on some pain than others.
Still pretty good for back pain, though. Beats any non-opiate.
Indica, of course. Or a high CBD stain.
However, a Sativa can be quite useful for doing stretches and exercises. Keeps you focused on them for an extended time and you still get that wonderful "getting in touch with your body" feeling which can be damn useful when relaxing spasms and pain.