It's really good... when they let you use it. I've been experimenting with using Opus 5.5 to reverse engineer an early 2000s disc cloning tool, but unfortunately the built in “classifier” went overboard and poisoned the session, making it refuse to do anything further. Switching to a less powerful model didn't help.

Write a “hand off” document? Prohibited. Focus on something else? Prohibited. Eventually every single response was terminated by the classifier before it even began, with the reason being a placeholder string like <thinking quote> or similar. Before it totally forbade any form of response, it agreed with me that it was unfortunate, but stood proudly by its guns.