Like Fright Fest, It gets pretty crowded. If I recall correctly, Cedar Point started there Halloweekends in September last year. Lastly, HALLOWEEKENDS IS WAY BETTER THAN FRIGHT FEST. It is more scary, graphic, and there are so many scare zones throughout the parks where as we only have Necropolise which isn't even scary.The time and effort put in to Cedar Fair's Halloweekends is truly amazing.
Halloweekends is on par with fright fest, though I'm told the haunted houses are far superior and included with general admission(or season pass entry if you happen to have one).
Halloweekends has been going on for quite some time, probably at least 6-7 years if not longer. If you go on a Sunday early on in Halloweekends, like before the 3'rd week of October, it's usually pretty dead even on the off chance that the weather is nice. Fri's and Sat's tend to be pretty packed though. If you go on Sun you can pretty much expect to ride every ride in the park as many times as you want. For example last time I was there on a Sun last year, MF had on average about a 5-10 min wait, TTD 10-15 mins if that and everything else in the park was pretty much a walkon or 1-2 train wait.
Favorite Wood Coasters: The Voyage, Ravine Flyer II, Thunderhead, Balder Favorite Steel: Voltron Nevera, Steel Vengeance, Expedition GeForce, Olympia Looping Parks visited: 232, Coasters Ridden: Steel: 894, Wood: 179, Total: 1073
Halloweekends usually begins around the 3rd weekend in September, which is nice because it answers your second question. I've been there in September and October. It tends to be less crowded the earlier in the event you go. If you can manage to go there in September, you are probably better off.
Likewise, I've went mid-October and could barely make it through the midway without being shoulder to shoulder in a sea of people. It would seem as if CP either a) has no set capacity or b) has a high enough capacity where everyone is allowed in because my friends and I managed to ride the train around the park for about an hour or so because there was no way we were going to wait in lines or have to fight the crowds of people just to get through the park.
Does it compare to FF? I'm not sure "which one is better" because I believe that each event has their own unique attributes that make them stand out. For instance, I feel that the haunt attractions are better at CP but the shows/entertainment tend to be of better quality at GAm.
Cedar Point is technically the only park that has Halloweekends however all of the other Cedar Fair Parks have Halloween Haunt, except for Michigans Adventure which is closed in October. The old Paramount Parks event was called Fear Fest, however when Cedar Fair bought Knotts Berry Farm and got the rights to Halloween Haunt (the first theme park Halloween event which started in 73) so they decided to re-brand all of the events except CP's under the Haunt banner.