@Thomas,
This was a big part of my studies in getting a special education master's, dated by now to be sure but what I've kept up on seems to indicate that what I learned then still holds true.
The kids with disabilities tend to do better than if they were segregated amongst other kids with disabilities, the kids without disabilities tend to do as well as they would otherwise, but the kids without disabilities tend to score higher on measures of empathy than kids who don't have to deal with disabled kids.
It really seems to be a rare win-win situation (as long as the teachers are aware and prepared, which as boomer indicates not all are, unfortunately).
An exception, as you know from what I've talked about before, is Deaf kids. That's a language issue more than a disability issue, though. (Deaf kids have complete access to ASL but very limited access to spoken English, and need a critical mass of ASL to be in an effective learning environment.)
Oh one more exception is the more extreme behavioral disabilities.
The "best" mainstreaming seems to be pure physical disabilities like kids who need to use wheelchairs or who are blind, developmental disabilities like Down Syndrome, some types of autism, and learning disabilities. (Probably forgetting others.)