The reasoning is great in opus, unbeatable at the moment.
I understand what you mean, it becomes disappointing on more niche or specific work. It’s honestly a good thing to see these models are not really intelligent yet.
I use it for reviewing existing code, specifically for a components-based framework for Godot/GDScript at [0]. You can view the AGENTS.md and see that it's a relatively simple enough project: Just for 2D games and fairly modular so the AI can look at each file/class individually and have to cross-reference maybe 1-3 dependencies/dependents at most at any time during a single pass.
I've been using Codex, and it's helped me catch a lot of bugs that would have taken a long time on my own to even notice at all. Most of my productivity and the commits from the past couple months are thanks to that.
Claude on the other hand, oh man… It just wastes my time. It's had way more gaffes than Codex, on the exact same code and prompts.
I still get really mad at AI sometimes and I am not sure whether I could use AI for coding full time.
(Codex broke my git a few days ago.)