I have a Claude $200 (Ultra these days) plan and use Opus 5.5 for work with ultracode workflows and all the rest every day. I also use DeepSeek V4.1 Flash 1107 through a separate subscription plan with Pi.

Using DeepSeek I've made the Wayland compositor I use every day. It's finished now in the sense that it doesn't crash or have any bugs, nor can I think of any features to add or anything to change.

When somebody else is paying, I'll absolutely just throw Opus 5.5 at it. Who cares. It gives the best chances that the output is what I want, at no extra cost to me. But for my personal projects, using DeepSeek in Pi with a workflow extension is competent.

Sorry for throwing lots of text at you. All of this to say there's more to it than just which model is more intelligent. There's also speed, cost, and "good enough". Perhaps the problems I'm working on right now are just not complicated enough for "Opus is better" to be a real selling point.

Have used 5.5, ran usual battery of tests, and the downhill death spiral in a science/chemistry context continues. Code wise, it is better than open weight in several ways, but you really have to work hard to fight it from creating such a heavily abstracted mess it becomes unmaintainable for humans. Requires too much handholding for me. It's scientific reasoning is more confident as well, which would be great if it were also more correct.

Open weight models are generally more humble when working at fronteir, seek human input more reliably on thi gs they have no buisness deciding on, and can be trusted to not fuck up codebase in some way. Also, the way claude talks by default and tends towards even with all the "use this standard or that" tricks, grinds my gears in a way that drive me postal.