I was skeptical about spending money for a gizmo designed to store fresh herbs. Why not just put them in jars of water in the fridge, right? But if your refrigerator is as crowded as mine, that method can be pretty cumbersome. That's one reason to buy Prepara's Herb Savor. Another is that it really does extend the life of your herbs. I put one batch of mint in a glass of water with a plastic bag around it, put another in the Herb Savor, and put both in the refrigerator. The Herb Savor kept the herbs fresh for three weeks, while the herbs in the glass wilted in about a week.
Here's how it works: you put a bunch of herbs (or, say, asparagus) inside a plastic container; this goes in a "water well base," which you refill periodically. Just under 12 inches tall, the Herb Savor fits inside a refrigerator door. Progressive International's Herb Keeper is cheaper than the Herb Savor, but you have to unscrew it to replace the water. Between these two in cost is the Herb Keeper by Cuisipro, which has a suspended tray that lifts herbs in and out of water. I think all three are an improvement on the water-jar approach.