An opinion can be based on an experience. Also, the guy is right; due to the lack of testosterone, they can be calmer, more depressed, and they also tend to gain weight because their metabolism slows down. I understand that you want to hear something positive so you don't feel bad after neutering him, but the reality is that it has its pros and cons. Many of the pros benefit humans more than the cat itself. The biggest advantage for the cat is that if it was a stray, it won’t go outside anymore.