Depending on where you and who you ask, you will get large varying reviews.. I have never been to Sibbald, so I'm just giving my option based of reviews good and bad if others..
First off, it is one of 3 Provincial Parks right on the edge of the GTA (Bronte and Earl Rowe being the other two). So it gets a lot of City people, specially on weekends, from all the reviews I have seen, this means there is a high possibility of it being a party park for city folks.. Higher possibility then say Grundy, Emily or Parks further from the GTA..
Some reviews people claim the OPP (police) are at the park constantly , some say they have a station within the park just cut down on rowdism. Again having never been, I can't confirm that, safe to say that by all accounts there is a police station very close to Sibbald.
Now, would I go to Sibbald, yes I would, but timing is everything.. I would go weekdays during the summer, park will be quieter, less GTA partyers. From people I know who have been there, they go on weekdays in the summer and weekends in May, June, September and October.. excluding any and all long weekends.
Not sure this helps you much..
I can confirm what Snow has described above. I have personally been there twice, from Sunday to Tuesday. It is night and day when you compare Sunday to Monday and Tuesday. Overall it is a very nice park, but stay away on the weekends unless you ARE looking to party.