Any reviews of the new GPTs?

As far as I can tell from the discussions/blogs, GPTs are specialized versions of Chat GPT-4 that users can create.

• Is it essentially a Chat GPT-4 with a huge quantity of "custom instructions" that tell it how to respond? (More than the ~1500 character limit users have now.)?
• Aside from filtering Chat GPT-4 for special use cases (e.g., "You are a math tutor...") is there any added benefit beyond having bookmarked "flavors" of Chat GPT-4 for different tasks or projects?
• Has anyone found that it performs better than vanilla Chat GPT-4 (or "turbo")?
• Has anyone any further tips about what to type in to the builder for better performance?