I gave it a spin with instructions that worked great with gpt-5-codex (5.1 regressed a lot so I do not even compare to it).

Code quality was fine for my very limited tests but I was disappointed with instruction following.

I tried few tricks but I wasn't able to convince it to first present plan before starting implementation.

I have instructions describing that it should first do exploration (where it tried to discover what I want) then plan implementation and then code, but it always jumps directly to code.

this is bug issue for me especially because gemini-cli lacks plan mode like Claude code.

for codex those instructions make plan mode redundant.