I used to take TKD as a kid and I hated it, but saying it's useless and a shitty martial art is not giving it enough credit.
The reason people think it's shitty is because they see the "sport" version of it (like the one I took). However, "real" TKD like old-school styles are very different. My Muay Thai trainer is a black belt in TKD from an old-school dojo, and he said most of the masters were scarred up and badass fighters who did full-contact training. Hell, I've seen some middle-aged ladies at my gym do a "sport" version of Muay Thai for the cardio, so someone could get the wrong impression of MT if they watched them.
That being said, I personally like Muay Thai better because it teaches very effective punch/kick/knee/elbow combos, it's fun as hell (especially when taking classes with friends), and you don't have to memorize forms to pass tests. God I hated that in TKD.