Tea

If you like it, yay. If you don't like it, nay.

Depends on the tea, really. It can be a nice addition in the winter.
We're already adding milk, why not something else too?

I wouldn't add a stick of cinnamon - it's way better freshly ground to make proper masala chai :)

Cinnamon is often a big component of masala chai, so I don't see anything particularly controversial about using it in tea as a sole spice. I can see it working with a heavy, malty tea like Assam, especially if made "Indian style" with simmered milk and sugar.

If I have chai tea then I put extra cinnamon in. It's got a sweetness I love. Lots of extra cinnamon and milk is perfect!

Masala Chai is delicious. In the middle east, most teas have a single added spice such as cardamom, bergamot oil, and mint in the case of Moroccan mint tea, as well. Sure China and Japan don't like adulterating their teas but they have a sudo-religious relationship with tea that has many strict rules.