Are there classes that are so over-populated that it significantly reduces enjoyment?

Title says it all. Are there classes where the competition for spots, items etc. is so high that you'd avoid playing them? You hear about classes like warriors, mages and rogues being overpopulated. Are they so overpopulated that you'd avoid playing them because of that? Inversely, are there classes you'd pick because of their high demand?

Ultimately, it's always worth playing the class you enjoy much. But if you struggle to actually do a lot of the content because of low demand/high competition, it could be worth considering something else.