Personally I think they are useful, but in a much narrow way than they are often sold as. For things I'm very familiar with, they seem to reduce my productivity by a good chunk. For things I don't want to do like writing some kinds of tests it's probably about the same, but then I don't have to do it, which is a win. For things I'm not very familiar with it probably is at least 2x faster to do with LLM, but that tends to diminish quickly. For example, I recently vibe coded a website using NextJS without knowing almost anything about it. Incredibly fast to get started by applying my existing knowledge of other systems/concepts and using the LLM to extend it to a new space. A week or so of full time work on it later I'm at the point where I know I can get most things done faster by hand, with the occasional LLM detour for things I haven't touched before