Convenience. The Keurig K-cup adapter was easiest to use, though you have to wash it between uses. The Simple Cups product was hardest to use: Its lid was especially tough to close. Brewing time.

Keurig machines are a great way to make coffee fast, but coffee isn't the only thing these gadgets can do. For one thing, in addition to coffee, you can buy K-Cups filled with tea, fruit juice, or ...