I have a English cocker spaniel (show type) - he was neutered at 3 years old. I didn't want to do it too early because I wanted him to have fully matured both physically and mentally.
The main reason I got him done was because other male dogs at the park were attracted to him and were challenging him, he's not a submissive dog so would end up having a pop at them. Nothing serious just a lot of noise but I didn't want to chance a proper dogfight. Never had any issues with him marking in the house or humping.
A couple of months after the op, I noticed he's a lot more chilled out with all other dogs in general at the park. Dogs that would regularly have a go at him now didn't care less about him. His recall actually improved, although he can still be very stubborn when he wants to.
Negatives - his coat unfortunately did change - before it was very long and silky and a few months after, his coat sort of blew up and went all woolly. He can't be handstripped anymore, only clipped. But it was a small price to pay to have a calmer dog. He also became even greedier, before we could leave food around and he probably wouldn't touch it, now you have to be very careful. He never turns his nose up at his dinner now though, if he does we know there's something seriously wrong! He did actually lose weight after neutering, as opposed to putting on like some dogs do.
He was back to his normal self about a day after the op after the sedatives wore off.. do have to be careful about them running/ jumping up onto things as you don't want the stitches to open up. Took about 2-3 weeks before the wound was fully healed. They do bounce back extremely fast.
On the whole it was the right choice. He's always been a very confident, good-tempered dog, neutering just took the edge off when it came to territorial males. Shame about the coat, but he still looks just as handsome!