I'm a Cedar Fair fan. So far, I've been to:
SFMM, SFGADV, SFMW, SFNE, SFDL, SFA, SFWOA (pre-CF), and The Great Escape. For CF, I've been to CP, KBF, Dorney, then all of the US Paramount parks pre CF.
I'd say that Six Flags parks have slightly better roller coasters. But, Cedar Fair keeps their parks are more reliable (ride wise).
For my experiences, I visited SFMM in 2003. X was currently down, along with Flashback, and about a dozen other flat rides. Deja Vu lasted just long enough for me to get a ride before that went down. The park was very dirty, the people (guests and staff) very unpleasant. I was there with my mother and she was called a lier by the staff (they were accusing her of cutting line while waiting to get season pass, despite all the people around her saying she didn't). There were many parts of the park that looked neglected and it wasn't all that pleasant.
SFKK (2005) was an absolute dump. I can't think of a single good thing there, I only stayed long enough to get my coaster credits and got the heck out of there.
SFMW was decent, but when I visited in 2001, a lot of the aquatic "stuff" was empty, etc. There weren't too many rides other than the main coasters. Too much cement, but overall enjoyable.
SFDL. I liked this park, I liked the rides, it had a lot of shade, a nice smaller park. But again, the staff were incredibly annoying. The last time I was there I had sunglass on while getting on SROS. I had ridden it just hours before with sunglass and no problems. But that time, the ride attendants told me to take them off. Ok, I put them in my hand. Not good enough for them. Alright, I put them in my cargo-pants pocket and buttoned them up, STILL not good enough. The guy told me to give them to him or he would kick me off. Talk about inconsistency and rudeness. A similar thing happened at SFMM know I remember.
SFGadv. I love their rides. Kinda Ka is as good as TTD, I love Nitro and Medusa, GASM is one of my favorite arrows, and El Toro is AMAZING. On the bad side, every time I've been there (4 times), there are always many rides down. I think every time i've been, either 1 side of Batman & Robin were down, or both. Rolling Thunder seemed to always be down. Mine ride too. Dirty, very neglected areas, understaffed, untrained staff, and unfriendly staff. The last time I visited (opening year of El Toro) there were sooo many problems. They didn't open El Toro or Kinda Ka until noon each day. WHY? Thats just unacceptable. On top of that, Kinda Ka was unreliable all day, and they only used 1 station both days I was there. Both Rolling Thunder and Batman & Robin were closed for what seemed like most of the summer. The park was noticeably understaffed. I remember waiting in line for a hamburger for nearly 45 minutes because they didn't have enough people working. On top of that, they also didn't even serve several items listed on the menu (that some people waiting 45 minutes just to get). I was with my family, my mom ordered a salad that was on the menu, the employee didn't even know they served salads! And this wasn't opening day, this was in August. He had to consult other people to solve the salad mystery, including asking my mom what goes in a salad... To make matters worse, they charged us extra, charging us for 2 beers that, get this, they didn't even have, yet it was on the menu. When I went back to show them they overcharged us, myself and my mother were called liars! So far a bad day, huh. Later that day while in line for Kinda Ka, I heard someone say that El Toro was already closing. It was only 8:30 so I thought the guy was just confused. We'll, after getting off KK at 9:00 and walking around, It became evident that ALL the rides were closed. The park had decided to cut back their hours due to money problems. Only thing is they neglected to tell ANYONE. Not a single person in the park new this. The park maps, website, entrance, etc all showed the normal 10:00 closing time. Talk about a way to get people out of your park... So yea, that was pretty aweful.
SFNE, my home park, a decent park, but again the staff isn't well trained.
SFA, a dump.
SFWoA. Decent park, but a lot of the rides/coasters were closed when I visited, the whole back of the park was shut down, etc. Not a fun day.
The Great Escape. I enjoyed that park. Not very notable, but decent.
I don't even think I need to write reviews/experiences on CF parks. I have been to CP many times, the only time a ride was down was the opening year of TTD and during the big ACE event there. But I got to ride it so I'm happy enough. Its the best park I have ever been to and is almost picture perfect. KBF is a great family park with great rides. Same goes for Dorney. Friendly staff, well trained, well staffed, and the parks were taken care of. I visited Geauga Lake when CF took it over and in just its first year they did a FANTASTIC job. They started rebuilding the water park, started opening up the back of the park, cleaned it up, and really put life back into it. Not the best rides, but it became a pleasant park to visit.
I've been to all US Paramount parks, pre CF, and I enjoyed them all. I liked the staff, rides, lots of trees and scenery, well taken care of. I'm guessing that CF has done a good job to keep the parks beautiful.
I'm sure SF has done a lot to try to improve their parks, but from my experiences, SF has been a nightmare and CF the dream.