Hi,
I've been shooting slings pretty much daily for about a year now.
A few thoughts :
1) The power is impressive. An egg sized (and shaped) rounded stone can deeply puck an empty 45 gallons drum. An inch wide steel ball goes through one side of the same drum, and pucks the other side. Small stones can cut tree branches just like a rifle would (the smaller the stones, the faster they travel). When you hit a stone square on a boulder, it explodes (the stone, not the boulder

)... Be careful with those "toys"...
2) The accuracy requires a lot of consistent work. After a few months, you end up being able to hit a man sized target at 50 yards pretty much consistantly. After a year or more, you can hit a soda can at 30 yards pretty much regularly (of course there are misses, but you can hope to get it). You need to practice daily, but not a long time each day (it's often hard to stop, actually

). Start with painfully easy targets. Hit them, and think "this is too easy". When you hit your target 5 times in a row, step back a yard. Hit the target again, etc. This will build up confidence and help you align your aim gradually.
To prevent some injuries : start your practice session with a warm up (think baseball pitchers -- start with some warm up exercices, than slow and easy pitching, with light stones. Increase power very progressively, only when you feel warm).
3) I wouldn't hunt big game with a sling (but I know someone who did it successfully), but for anything fox-sized or smaller, it's really deadly. I've killed many crows with mine, and a few rabbits. I reckon a head shot on pretty much anything (deer, human, large dog, etc.) creates at LEAST a K.O., if not serious cranial damage/death. Be very cautious with slings, especially once you've started building up some power. Misfires and weird rebounds are dangerous for anyone around (that's 360°).
4) The power doesn't come from the rotation of the sling, but from the pitching motion itself. One complete revolution is all you need to propel a projectile.
Cheers,
David