I always say do your own benchmarking for your own use case.
The risk of a colored benchmark is quite high when benchmark is done by owner of product or by "fan" of product. With the exception of a well explained, clear benchmark that everyone can understand and reproduce easily.
At least with a public benchmark, people can point out flaws. With something rolled out internally, you're still likely to get flaws, but no one will point out that you misconfigured Postgres or set up Mongo the wrong way, or any of the other errors you are just as likely to make by doing it yourself.
Perhaps, once you've winnowed the choices down to just a few, it might make more sense, but I think good public benchmarks can be a helpful thing for that selection process.
I agree, public benchmarks do have their uses as i noted in my reply:
"With the exception of a well explained, clear benchmark that everyone can understand and reproduce easily"
An internal benchmark indeed requires knowledge on the subject, but most of the times people at the mailinglists are quite helpful when you explain what you are trying to do. Especially when you post a benchmark which is not in favorite of their product.
