GM destroyed a lot of its brand equity in the '70's and 80's with Vegas, the infamous X cars, the J cars of the early '80's, etc. It became clear that there was a large segment of the population that would not consider a GM brand under any circumstances -- the "import intenders". If you're in the automotive business or even read the comments on USA Today you'll know that this is true.
Saturn was conceived to get these buyers who would not normally even go to a showroom and by that measure, it was successful. It is surprising how many people -- even within GM -- didn't realize that Saturn was a part of GM so this helped avoid the GM stigma.
This "import intender" problem still exists today -- for evidence, look at he few number of Matrix buyers who actually have Vibe on their consideration list even though the products are basically the same. Try selling a Civic buyer a Cobalt.
In the end, GM wasted this opportunity by first starving Saturn of new product, then by trying to bring it into the GM fold. Companies often have divisions operating independently to create competition and foster innovation and expertise but management chose a different direction for Saturn.