L-Theanine is somewhat sweet and... chalky, perhaps. It's a little savory/funky too. Not altogether an unpleasant flavour. I occasionally take a teaspoon of unpacked L-Theanine powder and stir it into a glass of water. It doesn't mix very readily, but once it does, it's barely noticeable (at least to my palate). I don't notice it when I repeat this process with green or black tea, so I would be surprised if anyone could detect it in coffee.

It doesn’t really taste like anything. Biggest thing is that you need to stir it quite a bit more than something like sugar to get it mixed

Although slightly higher in cost, you can get Theanine in pill form.