Why is it good shape?
Eye space and efficiency.
First, let's look at the marvelous example of the ponnuki in the corner, the coolest living shape in go (IMHO).
White stones are here just to show how pinned in you can be and still be a-ok.
Next, one of the most common and powerful locations for a ponnuki, the edge. Although not alive by itself, it is a complete eye. The lower gaps are safe because of the edge (opponent must approach). The upper edges aren't, but they are effectively miai -- you can lose one as long as you have the other.
I was going to do a separate eye post (as a Beginner Tactics), but haven't done so yet. It helps to remember this basic definition though:
The chain of stones surrounding eyespace must have no more than one enemy stone occupying it.
Bearing that in mind, you can see that the ponnuki on the edge is ok as long as once one of the upper corners is taken by an opposing stone, you fill in the other (or verify it is cut-safe). Obviously we assume you will block peeps at the edge.
Even ponnukis in the middle of the board are strong, but far from guaranteed eye space.
So, in theory, these are not yet solid eyes because maybe the opponent could cut into more than one corner. Despite this it's still a rockin start at getting an eye, and will almost certainly end up being one.
These are completed eyes based (again, barring double plays) on their being only two points left which are miai. But, at this point, it should all be fairly obvious, yes?
The best thing about the ponnuki is that it is not just powerful, but cheap.
At most it's 4 stones, but really, since a ponnuki often forms after capturing a stone, it's often effectively only 3 stones in cost.
Don't Send Ponnuki-shaped Valentines Just Yet
Of course, ponnukis have a seamier side as well. They often form in and around ko fights. Often a hanging connection is formed in the corner above the edge just to pick a fight. Can you believe that?