According to the vet, Simba did very well with his neutering today. He's never very happy to go to the vet anyway, and I'm sure after today he really won't want to go again. But everything went well and he's back home recovering. We've got some anti-inflammatories to give him to help keep the pain down, some mild sedatives to keep him calm and mellow while he recovers, and an e-collar on him so he doesn't try to lick his surgery site.
He should be back to normal in about a week or so. In the meantime you can bet he'll get spoiled even more than he usually does.