The design is simple: it's basically a hanging planter with a hole cut into the bottom and a tomato plant pushed through the hole. On the top of the planter you can put anything you want: flowers, herbs, or other small plants. I'm using oregano, because it can be easily grown in a pot this size, and it makes a wonderful seasoning for the tomatoes in a fresh garden salad.
The tomatoes I'm using are a variety called "Supersweet 100". I've grown these for the past few years and they are just delicious -- sweet & tart with a crisp bite. They aren't very big, about the diameter of a penny, but that's fine for our purposes. Smaller tomatoes do better in these planters than larger ones, which require more soil and space. Also, larger tomatoes tend to sag towards the ground, which puts stresses on the stem of the plant.
Properly watered and cared for, a planter like this can yield a great harvest. Last year I picked over 150 tomatoes from a single plant! And you can reuse the planter year after year!
Regarding planting time, you should follow the instructions that come with the plant, usually located on the back of the little plastic tag that indicates the variety. The nice thing about these planters, however, is that if you start them a little early you can just hang them inside until it's time to bring them outside.