If you mean is it a good earner - it is for me. I have sites built around Google CSEs, and they quite well.
If you mean is it an effective search appliance for a site - well, sorta. Depends on the site. I have had problems with it on some sites (where we're only using search for the site pages themselves, no off-site pages included) because I can't weight the search enough to influence the order of the pages.