It depends on both the target language (in that the quality of what's generated is variable, and the benefit it gains you is also variable based on how easy it

It depends on both the target language (in that the quality of what's generated is variable, and the benefit it gains you is also variable based on how easy it is to get something usable in that language), and how well they defined the API using the spec. I've had a few times where the spec had slight inconsistencies and bugs that made working through and debugging the generated output take as long or longer than it would take to implement a client from scratch for the 20% of the API I actually need.