The greatest actual danger with illegal drugs is that you never really know what you are going to get. The naturally occurring magic mushrooms are picked out of cow shit. The farmed ones are subject to contamination, maybe the grower had a contaminated batch but just picked off the mold and sold them to you.

DMT is a molecule that is practically identical to Psilocybin and other than duration and method of consumption, I doubt most people would be able to tell the difference in a double blind test. Under one of the more bizarre loopholes in US law, DMT is legally available in unextracted plant form. To be orally active, it must be mixed with an MAOI, also available in plant extract form. However, if you possess the knowledge and intent to use the DMT containing plant for psychedelic purposes, you are committing a crime. It is perhaps the truest example of a thought crime codified into law.

My personal experience with mushrooms was very helpful -- I gained the ability to completely clear my head of thoughts and maintain that state. Each individual's experience is personal, of course, but achieving that bit of zen has been great for me. If you don't already have total control of your active mind, you may be able to fine tune it with the aid of such tools.

One negative could be wasting time. Since you're asking this I doubt you'll be the type to abuse psychedelics, but one is completely unproductive while tripping. So any benefits one gets diminish rapidly with each successive trip. One time won't hurt you cognitively at all, but 100 trips will. You won't get HPPD doing it just once, but it becomes a real possibility when these drugs are abused. http://en.wikipedia.org/wiki/Hallucinogen_persisting_perception_disorder HPPD is not the same as flashbacks, flashbacks are BS they tell kids about in DARE. Hallucinogen flashbacks are only occur when someone has a trip bad enough to suffer PTSD. That doesn't happen very often. I would be surprised if it has happened 100 times in history.