These machines use little plastic pods filled with coffee or tea leaves. I'm not a coffee drinker but I've heard from my husband that they make an excellent cup of joe, delivering it quickly and piping hot. In fact, the machines are primarily made for coffee but an ever-increasing line of tea is also available. Twinings, Celestial Seasoning and Tazo (Starbuck's house tea brand) all have offerings for the Keurig.
On a whim, I tried them out. The machine didn't deliver a bad cup of black tea, although it was a bit weak. My guess is even with the hot water flowing through the k-cup, the tea didn't have time to brew properly so the full flavor didn't leach from the leaves. This was most disappointing in the Earl Grey, since it didn't develop that heavenly aroma I'm used to.
The green tea, in my opinion, is a total bust. My guess is the water is too hot. A Keurig brews at 192 F, while green tea is best brewed at a much lower temperature, generally near 160 F. Some Keurig machines allow you to adjust the temperature, but by no more than 5 degrees so they really aren't suitable for green tea and some herbal blends.
I now use their Keurig for the hot water when I am visiting, but I brew the tea the old fashioned way with a bag in a mug. I think Keurig machines do what they were made for, brewing coffee, well, but they fall a little short in the tea department.