My DC started PAC on their 3rd birthday. I loved it. I met other parents with kids like mine and my son was part of a group. Also, PAC really helped get the IEP and accommodations for kindergarten in place well in advance.
My DC was informally considered a peer model. Not being diagnosed Autistic but needing intensive language support, he showed typical emotional reactions, positive and negative, in social situations and loved routine while not being overly inflexible.
He's now in 3rd grade and no longer in PAC. Some of the behaviors did stress him out and made it hard for him to focus and his teacher determined that the structure and routine of the larger class was better for him. He gets support with a couple of HFA kids with an IA.
But he still visits his PAC buddies at the end of the day before dismissal, mostly because the SPED team at our school is awesome and knows he's been with some of kids since PAC and they enjoy seeing each other.