I have two kinds of those. One uses a metal filter so the cup ends up with a bit of sediment, and the other uses paper filters so it's more like a tiny Mr. Coffee machine. (Baby Coffee?) That's what I use most days.
If you have a Keurig 2.0 machine you need to use the pink adaptors. Keurig 2.0 includes an optical sensor that's supposed to verify that you're using an authorized K-cup. (I.e. the manufacturer paid the license fee and passed the cost on to you.) But a pink plastic filter adaptor fools the sensor.