I haven't looked into penny stocks in years myself. But when I did, here was what was wrong with them. A small investment can change their price much more easily than in stocks listed on the NASDAQ and NYSE. Also, they often have a higher expense ratio compared to what you are investing (this was several years ago, might have changed). Also, the data I could get on them was very limited. Although some data suggests that they might be quite profitable, the same data admits that it does not contain those companies that went bankrupt (so it is very questionable).
Personally, I would say to stay away from penny stocks. While it is true that some penny stocks have made their investors a ton of money, it is also true that they are generally seen as pretty risky. I personally would never recommend a penny stock to anyone.
I champion value investing, which provides greater profit, safety, and more predictability. My advice is to stay away from penny stocks.