They do make losers out of people who abuse them, fortunately with most drugs there's a big line between use and abuse. You're right that some people take drugs expecting that they'll suddenly be more creative, that's not true. Some drugs, like LSD and mushrooms, open your mind to new perceptions and new ways of looking at/thinking about things, but they won't make you paint a masterpiece or be the next Mozart.
In the case of hallucinogens you can look at it this way. Since birth your mind has created all kinds of filters for information to do things like seperate thought from reality, memory from perception, to make sure only what's useful or important reaches your conscious thoughts. Language plays a huge part in this, when you see an object your brain automatically attaches a meaning to it. You can think of language and your senses as a sort of barrier between yourself and the outside world, any information you get about reality must pass through them and it's inevitably "tainted" by them in the process. All of your experiences, memories, emotions and knowledge play a part in interpreting reality, which is exactly why art can have completely different meanings and interpretations for different people. A painting never changes, but what it means and represents changes for every person who looks at it.
Hallucinogens temporarily dissolve this barrier to a certain extent. If you take enough of them it dissolves completely, you stop distinguishing between yourself and the world around you. Ego disappears completely because everything that makes you who you are, your memories, experiences, personality, emotions... none of them get applied anymore. Nothing has a set meaning anymore and you feel for the first time in your life you are seeing the world around you as it's meant to be seen, without any intrusion from your ego. This is where a lot of people freak out, because while your ego is fading away you could swear that you're dieing, and in a sense you are. Everything that makes you an individual fades away until you're left with the notion that you are everything around you, without any seperation. Most people describe it as a feeling of waking up for the first time from a life long dream.
Then, a few hours later, the chemical leaves your body and everything goes right back to normal. Except you've had this experience now that you simply can't get anywhere else, an experience you'll likely still be thinking about and trying to digest 6 months later.
That's about the best I can describe it to someone who's never experienced it for themself. I mean, it's like trying to describe sex to a virgin, you can talk all you want about how incredible it is but at the end of the day it's something you just won't get until you try it yourself.
