Some are calmed down, causing them to be less vicious, stops the humping on human's legs, and every other animal they see. Some get so lay back that they get fat if the owner/handler doesn't keep them well exercised. It is still the best thing for them health wise.
Will your vet be doing a vasectomy, or total removal? I've been trying to get my son to neuter his boy, with no luck. I've even told him about the new surgical procedure that I was told about, which leaves the dog looking intact, but have no knowledge of it first hand.
IMO, No rush... You can do it later on if you really want to. My dog had it done when she was about 4 years old, after her first birth. Shes 7 now and shes still just as active and hasnt changed one bit. However, you will see a huge change in them right after the surgery of course but after they recover everytihng is back to normal again.
And yes they do gain weight if you dont keep them active...
It is ezer on the dog and may cost more when the dog is older. It happen to me with my bull dog. If u are going to get it done do asap. And it dont really effect them. Mine acted the same way.