I have smoked most of my life, and quit recently in order to get some personal issues in order, and I can tell you a few things for sure.

1. You will not become physically or psychologically addicted with casual use.
2. Some times the first time people smoke they don't feel much. I have heard of this happening before.
3. You will not see visual hallucinations smoking small amounts of marijuana. If you ingested large amounts tho you may see small visual distortions.
4. I was never big on other drugs, even alcohol, and marijuana was my choice. I think most people see a clear difference between marijuana and so called hard drugs. Furthermore your approach towards it and your behavior is more important than the act itself.

My suggestion would be to give it a try. The human brain has built in receptors specifically evolved for cannabis compounds. Humans have been eating and smoking it for thousands of years. Unless you have severe psychological issues or a family history of schizophrenia, you have nothing to worry about using it casually. I would suggest being careful if you are on any medications as well because it can multiply the effects of other drugs. For anyone who has any questions about any drugs I would suggest www.erowid.org, it is a wonderfully detailed resource.