Is the purpose of these tools really to spend less time? I think their main value is reducing mistakes through having one extra set of eyes, even if mechanical ones, looking at the code.

As a sole developer of a non-trivial open source project, I've recently started using CodeRabbit, very skeptical about it, but right on the first PR, it actually found a bug that my CI tests did not catch, decided to keep it after that.

Gemini Code Assist on the other hand, the first suggestion it did would actually lead to a bug, so that was out immediately.