I have used Klean Kanteens extensively for boiling water. As for being designed for it, they do not suggest them being used as your every day pot but Klean Kanteen does note on their website that it is perfectly fine to use them for cooking in. The only issue that I have ever had is that soot builds up and needs to be cleaned off. I scrub mine down with a brillo pad when I get home and dont have any issue with the external discoloration from the heat/smoke. After using mine extensively the inside surface looks brand new and the material shows no signs of obvious stress. Feel free to check out my blog which has an entry on them.