Negative: You'll be driven mad seeing all the people clearly wrong about what you now know who are uninterested in actually finding out the facts yet unashamed in spouting off their misinformation.
You might want to elaborate on that question a bit. Do you mean for an individual, for a company, a government, a society in general?
And what do you mean by using the internet? Obviously unless you live as a hermit in the woods many systems you use every day are going to use the internet anyway, the traffic light as you are crossing the street, the order system the stores you shop at use to fill their shelves, your employer, the government departments managing the infrastructure in your area and your taxes and other bureaucratic data about you and your possessions, your doctor, the producers of most of the goods you use, the phone system,...
So presumably you mean some sort of direct use on a device you interact with, what do you mean there, the web? Social media? Messengers? Email? Voice calls? Video calls? Video conferences? Online games? Search engines? Mapping apps? Wikipedia? Youtube? Netflix, Amazon Prime and similar sites? Twitch?
Positive: you could make a website about anything, for example I used to have a bookmark to Amish Rake Fight GFY, which was just a single page with a "🖕Go Fuck Yourself" message and a counter of how many people were told to go fuck themselves.
Negative: Sadly, this is gone now and all I'm left with is fucking my memories. Web 2.0 sucks.
Positive: Government and corporate injustices are increasingly hard to sweep under the rug, and awareness of these injustices is more prevalent.
Negative: All this access to injustices going on can easily lead you down a spiral of hopelessness if you let it, especially when it seems like nothing is being done to fix the problem.