Does the usage of Haki affect the user in any way? We know that Haki is a mysterious power present inside every individual. But only few people are able to awaken and use it. It is very closely related to the willpower of a human being. So the question which arises here is: Does the use of Haki affect the user (physically or mentally) in any way? Does excessive use of Haki take a toll on the user?


If it is so, how exactly does it affect the user, since Haki isn't something like a physical energy?
If it isn't so, why don't the Haki users use it all the time? Why do they use it only in specific cases like a battle, or entering enemy territories, etc?
Does it affect the user (physically or mentally) who is using the Haki in any way?


No. I encourage anyone to point out otherwise, but my memory says Haki isn't proven to be exhausting. The conclusion I draw from that is that it's not a powerup (i.e. energy drawn from an inner pool of life force/chakra/ki/Namekian-hand-holding).


Does excessive usage of Haki take a toll on the user?


N-- but yes. By nature, excessive activity will generate stress but in accordance with my above answer, this is just an unlikely result.


If it is so, how exactly does it affect the user since Haki isn't something like a physical energy?


Haki is akin to concentration to activate, which also makes it task-orientated. Things like Observation are always on, because these are your most basic senses. As long as the user is conscious, there will be a heightened sense of plot convenience—I mean, the ability to detect hostile presences like the "wild beast" in Fishmen Kingdom's Palace. For Armament, this is used only for strengthening, i.e. intensifying the muscles. This is like getting in the zone, turning on that killer instinct, you know what you're about to do and you're gonna do it. So while the user can turn it on at the tip of a hat, there's no point to stay amped up and flexed. That's like walking around with a balled up fist all the time. Haki isn't going anywhere else. Nor is it a get-out-of-Death-Flag card that activates when your enemy has gone too OP or you're a pummeled mess. 

My train of thought: Haki users are just flexing and intensifying their basic characteristics of observation, density, and intimidation. So it is physical energy if you take all the potential energy and turn it into kinetic energy for battle. Since it's not a powerup but more of a concentrated assault mindset, the training makes more sense. It wasn't for the purpose of controlling the uncontrollable, but for developing this state that allows you to demonstrate more strength like knowing how to brace your forehead with hardening for a headbutt or tighten a fist to punch. Not to undersell it, I'm sure the power behind one Haki-shrouded punch will make the difference versus another eventually, but it's never been the deciding climax to a fight.