The pet communities that I'm part of are pro-speutering (a conglomerate of "spay" and "neuter"). You might think that I'd be against speutering, since it messes up the natural order of things. But I'm not, actually. I'd argue that speutering actually allows your pet's true personality to come out.
Why? Because hormones are powerful. Ask any woman with PMS. Or better yet, don't ask, and just let her be. Now just think of what they do to an animal with much less conscious control over its own actions--animals don't think so much as follow engrained behavior patterns that are laid down through eons of genetics and years of training. Hormones control just about everything in an animal, from its blood glucose levels, to hunger, to sex.
Removing the need to reproduce (and in animals, it's a NEED) allows the pet to concentrate on pleasing its owners (dogs), or cuddling on laps (cats). It lowers their tension levels, making them calmer. It's not a cheap way to get a calm pet. It's a good way to get to know your real pet. Let's face it--it's a lot easier to train a dog when he's not thinking "SEX SEX SEX SEX SEX", and that goes triple if there's a bitch in heat nearby.
One common speutering myth is that it makes the animal fat. This is not true. Sure, if you speuter your pet, it's not going to spend as much energy trying to have sex. But that just means you have to feed it less. Too much food, not enough exercise, is what makes pets fat--and with the amounts of food some people feed, I doubt neutering would have made any difference with respect to how fat the pet got.
Animals do not, in fact, change their personality after the procedure. If anything, their personalities become more clear, because they're not always going batsh!t crazy from their hormones. Shadow is a sweetheart, Boobies (yes, we actually have a cat named Boobies) sulks and won't love anybody but her cat-mommy, the Tweeb is a grumpy little old lady. Pokey is a sweet but fiercely protective Doberman. And so on.
And before you write me about the other two big points I've "forgotten": I haven't. Cancer risk and pet overpopulation are also very serious issues, but they tend to be more abstract and less tangible. We know, intellectually, that they are good things, but it's hard to know how much of a problem those issues are unless you are the one sticking animal after animal with euthanasia. But everybody (or almost everybody) has dealt with a yappy dog or a cat in heat before--it's not fun--which is why I chose to stick with the personality issue.