Ditto to some of the PP on fit. The collar should NOT be loose to the point of slipping down, it needs to fit comfortably but not loosely.
And yes, the prong is almost completely a self-correct collar. I've used them on 4 of our dogs--hoping to not need it with Odin, but still have two around just in case. After fighting your dog on a leash for a while it can be too easy to over-correct on a prong, so be careful not to.
On the other hand, it should make daily walks MUCH more enjoyable for both of you!
Chris, mom to:
Tank (Cardigan Welsh Corgi)
Odin (GSD 8/28/13)
Valentine-our ragdoll cat
Two great skin kids