The park was built by Newhall Land and Farm, a major construction firm based in the Santa Clarita valley, who built it up until it was a major tourist destination. In 1977, during a bout of financial difficulty, Newhall Land and Farm started fishing for bids to sell Magic Mountain, and it was soon purchased by theme park giant Six Flags. Premier Parks bought the entire Six Flags chain in 1998, not just Magic Mountain.
Magic Mountain holds the greatest appeal for coaster junkies, since it has a higher proportion of roller coasters to other rides than any other theme park in California. Most of the coasters that were built there were record-breaking at the time of their construction. Magic Mountain is currently building three new coasters (Deja Vu, X, and Goliath Jr.), and when they are completed, it will hold the record for the most roller coasters in a single park, with 15.
Snakeboy's claim that Magic Mountain has "gone to shit" is pure negativism. While it's true that the back side of the park (Spillikin Corner) is suffering from lack of attention, Six Flags is pouring more money into development for Magic Mountain than for any of their other parks, and considers it one of the jewels in its crown.
I lived in Valencia in the Eighties, and worked at Magic Mountain several times, doing different jobs. It's fascinating, because the park is constantly changing, with new rides and coasters opening every few years.