1) I smoked for years. The smoke coated my lungs and caused me to cough often. My vocal cords are permanently damaged such that I speak with a raspy voice even though I quit smoking years ago.
2) While high, my cognitive thought process was hindered such that I was slow to make decisions and the decisions I did make were usually bad ones. I (as well as every one of my friends who got high with me) could not carry on a truly intellectual conversation.
3) In Jr. High and High School I would often smoke before class. I was unable to fully comprehend what the teacher was saying for two reasons: a) the THC numbed brain function and b) I was easily distracted by fellow students and/or shiny objects.
4) Pot caused me to become lazy. Prior to my addiction to marijuana I was very athletic or generally active. When I got hooked I really didn't want to do much other than watch TV, sleep, eat, or waste time in some other non-productive way.
The above descriptions were not only experienced in a personal way but I witnessed very similar effects on my friends and acquaintances.
The following excerpt is quoted here:
HowStuffWorks "Marijuana and the Brain"
When I used to smoke pot I was one of its greatest proponents. Then, one day, reality struck and I was forced to face the truth. That's when I decided it was time to quit the harmful substance.