Why did many people have a bad first impression of GPT-5?

Actually, the reason behind that is absurdly stupid. OpenAI fucked up with UX. That's it. The model is actually good; all variants of it are. But OpenAI rushed the release for some reason, and their goal of making the UX better made it worse for a lot of users.

The key detail here was the model router that they added to ChatGPT so that users don't have to manually choose a model, and it can just choose the appropriate one on its own. For example, if you ask it how to pronounce a word, that can easily be answered with a non-thinking model, with lower latency and the same accuracy. But if you give it a math problem, ask something about coding, or just generally give it a task that requires more reasoning - it is better processed by a thinking variant of the model.

And the idea is good, especially for the average user who doesn't know much about how these models work and doesn't want to think about which model to choose for every query. But the implementation was very bad in the first couple of days, and OpenAI confirmed that themselves. The router was working poorly, not choosing a thinking model for complex queries when needed, and not only that, the information about which model answered the query was also hidden, so, for example, when your request (as a free/plus user) was routed to "GPT-5 mini", you couldn't know that. There's not even a "mini" model in the model picker.

And another factor is the "reasoning effort" parameter that OpenAI's models have. It determines "how hard" the model thinks before answering. If you need quicker answers, choose "low"; if you need more reasoning for more complex tasks, use "medium" or "hard". And the thing is that in ChatGPT you can't choose that setting yourself. It's part of the router, too. And the information about this setting is also hidden from users.

It turned out that most of the requests from free/plus users were processed either by a non-thinking model, or by a thinking model but with the "low" reasoning effort setting. And sometimes also by "mini" models when limits for the regular ones were exhausted. And the performance is, expectedly, bad under those circumstances.

So, even when paying users tried out GPT-5, they were often getting bad results. And that was their impression of GPT-5. And that's why there was so much hate for it online.

But OpenAI is fixing those problems, and some are already fixed. So, if you tried out GPT-5 in the first couple of days and didn't like it, consider trying it out again now or in a few days, as it might be much better.