Depends on how much $$, Amazon is selling them for $20....at that price its a great value. I also have the T20 which is a little bit shorter and also skinnier.
Previous poster was right about the yellow tint.... Mine is the most yellow tint out of all my lights, its not horrible, but it is noticeable and I don't like it compared to a nice cool white tint. With that being said, both the T21 and T20 throw a ton of light, the T21 is a monster flooder for it's size, it really does put out a massive amount of light for its size.
16340's really increase the output and fit "OK" as long as they aren't too wide. The T20 can't really take most 16340 because it won't screw down all the way and also causes some flickering on medium. No such problems on the T21, and the build quality is better too. Running lifepo4 in both yields seemingly equal brightness..... Which makes me question the 350 lumen claim... But since the T21 is floodier its hard to judge.
Either way, its an awesome light that for some reason doesnt get a lot of press. Also check out the newer thrunite saber 1c which is the T20 upgraded to xp-g2 emitter