Depends on the illness. This kind of behav mod works well for my Pervasive Developmental Disorder (PDD) kid and most of the time for my ADHD kid. My BiPolar (BP) kid, no way, no how.
In terms of speech, as Marguarite said, it's minute by minute work. But yes, mine went from no receptive or expressive language at 22 mo to very verbal now. However, my son still has apraxia and no amt of work is going to change that.
There's lots and lots of info on this site. Also, tons of books in the library on helping your child's speech. But absolutely you need to do this at home all day, every day.
http://asha.org/public/speech/emergent-literacy.htm
http://asha.org/public/speech/development/
One other note, in my experience it is very, very common for kids with speech problems to become frustrated with lack of communication and ack out. So would I probably. So if your behavior issues are solely related to speech then the behavior will resolve over time with improved communication. Also, mine pretty much read before he could talk. So if yours has the alphabet and is reading, while it's not good to use a crutch instead of learning, if it reduces the frustration, use a chalkboard or magnetic letters for communication. PECS is always an option as well.
