Few people remember that Pontiac was seen as an old-maid's car until the late '50s. Mostly they sold to people who wanted something a little nicer than a Chevy, but wouldn't spend the extra for an Olds or Buick. Bunkie Knudsen, a career GM exec promoted to head Pontiac in the mid '50s, changed that. Under his direction, they introduced the first Bonneville as a fairly exclusive two-door in '57, complete with an optional fuel injected engine (basically the Rochester injection used on the Corvette from '57-'65). Then they went "wide-track" and promoted performance into the '60s. Knudsen's program was so successful he was promoted to head Chevy, and John DeLorean replaced him at Pontiac.
DeLorean continued to push performance and stole the "GTO" name from Ferrari's famous 250 GTO. Under DeLorean, some of Pontiac's high perfromance press cars came through Royal Pontiac, a local Detroit area dealer famous for having a direct line to the factory for perfromance improvements. Sometimes this was reported, sometimes not, and the cars always proved very, very quick.
It's sad to see Pontiac go, but GM worked years to paint themselves into a corner by refusing to make a good, consisent line of small cars at one end of the market and continuously downgrading Cadillac at the upper end. So they've ended out with a compressed range running from basic mid-sized sedans to mid-level luxury cars. It's still a lot of cars, but not enough to keep six GM brands viable in North America.
At the lower end of the market people now buy bottom of the line Hondas, Toyotas, Hyundais and Nissans, while at the high end you see lots of S-class Mercedes, 7-series BMWs, and big Lexuses. GM really has nothing to compete at either end of the market. Billy Durant conceived GM to offer a car "for every purse and purpose", with the idea being that you'd start someone out with a low-end GM car and they've keep buying GM all the way to the top. It worked well, and probably would today if GM hadn't forgotten about the concept.