Saturday or Sunday would be better. You don't want to get stuck in Labor Day traffic both ways on 95.

We got fast lane passes when we went on a very busy day. They're not good on every ride, but it helped with all of the big lines for all but a few of the roller coasters.

I am sure this is one of their busiest weekends of the year, so yes, there should be huge crowds and lines.
If you're determined to go this weekend, maybe go Saturday or Sunday, be there right when they open, and it should be more manageable.

Has anyone been? My DH proposed it to the older kids who are now all excited. I'm wavering over whether it's a good idea to keep myself out of this, but then again it would probably help for me to be there since the olders and younger (6) would inevitably want to split up (olders being 10 and 11- too young to go off entirely on their own). The weather doesn't look bad, I'm just wondering about the crowds and lines. The kids start school the next day and I'd rather not them all be suffering from park hangovers.