I'm so often glad we included a pond in our landscape. A couple of years ago we enlarged it a bit with a short stream/waterfall.
Kim and I keep both koi and goldfish in the pond.
I won't get into the do's and don'ts of keeping both together. Suffice it to say that one "shouldn't."
The pond seems to add quite a cool feeling to the garden, temperature-wise that is. And there is so much activity which revolves around it; birds which come for drinks and baths, frogs, dragonflies, and the daily koi feeding.
Here is a short video of the koi/goldfish being fed. You can see how the koi (with little whiskers) push at the lily pads to get at the food.