Well, you already have me thinking I need to buy me one of every light in the top 10...
About your question: The circuitry is. The light itself er, 4Sevens said it was ok to run li-ions in the beginning, then went back and said it wasn't, due to the lack of metal to carry the heat away from the emitter and circuitry - so if you toast it, it isn't covered by the warranty. But around these parts, many run it on 14500s anyway, making sure to only run Max in short bursts of a minute or less
Think about it. 320 lumens at turn on... 300 lumens after 1 minute... From a light small enough to hang on your key-chain...
P.S. Tip for your wallet, don't forget the CPF8 discount if ordering from 4Sevens...