How does it work? You teach your child - abiding by your state laws - according to their learning style at their pace, offering as much repetition as they need to master a skill/concept. The child isn't confined by brick walls -the world is their classroom!
There are two wonderful homeschooling and Down syndrome groups:
Homeschooling and Down syndrome(generally very active):
http://health.groups.yahoo.com/group/homeschoolinganddownsyndrome/
Homeschooling Kids with Down Syndrome:
http://groups.yahoo.com/group/HKDS/
We have many parents who have pulled their children out of public schools to homeschool. I haven't heard much about what they are missing since leaving the system. For the most part, they are quite relieved to be done with it - not everyone has access to wonderful placements. Most therapies, if necessary, can be obtained in the private sector. Social activities are numerous in the homeschooling community.
The Riverbend site has some information about homeschooling:
http://www.altonweb.com/cs/downsyndrome/index.htm?page=home.html