Clean my thermos? Clearly you overestimate me.

But yeah, it's a complicated equation based on how hot you need your coffee stay, how likely you are to use a thermos, and how much you want to spend.

Note: when the press/thermos gets to the point of being embarrassingly dirty, Astonish: Clean and Revive gets rid of all the scum with no effort. It's overpriced so I don't use it very often, but that stuff is incredible when it comes to cleaning old coffee/tea residue.