Can't speak for SF in California, but as for the one in New Jersey. It a good park with some of the best roller coasters in the world.
We have two Six Flags parks in CA. Six Flags Magic Mountain, in Southern California, is the king of coaster parks.
Six Flags Discovery Kingdom, in Northern California, is the one that's closer to where we live. It's in Vallejo, which is about an hour's drive. Like Magic Mountain (and I'm sure several other SF parks), it was not originally built by Six Flags. It actually started as a park called Marine World/Africa USA and was primarily an animal park with dolphin shows, a zoo, etc. Six Flags bought it several years ago and started adding coasters. Just a couple of years ago, they changed the name. They don't have as many coasters as some of the other SF parks, but they do have some good ones. Medusa, in particular, is awesome. It is a floorless coaster, similar to Kraken at Sea World in Orlando.
The only thing I really don't like about SF is the clientele. You tend to get a rougher crowd that are there for the thrill rides. They tend to be on the rude side and not concerned with the families with smaller children.
Speaking only for the one in Vallejo, California, I was incredibly and pleasantly surprised to find that the employees and the clientele were great!! It had been about 7 years or so since I'd been to this park, and we decided this past Saturday to take a drive down there and ride some coasters. On the drive there, my girlfriend was telling me that the last time she went there, the place seemed really "ghetto" and that she didn't stay very long. I told her it would probably still be that way, but that we should adopt the philosophy that we're just there to ride coasters, and we could always leave if it turned out to be "scary". Much to our astonishment and surprise, we found the employees and the clientele to be incredibly friendly and polite. It was crowded, which tends to bring out the worst in people, and yet everyone was still great. We actually both commented on how friendly everyone was. I told her that when we got home I was going to post here and tell everyone how pleasantly surprised I was that the people at Six Flags were actually more consistently friendly than what I've encountered at Disney parks in recent years... and that I'd probably get flamed for doing so. :lol: