I actually haven't been since I started using my wheelchair, although I want to go at some point this summer(I live literally down the road). That said one of my friends went in her wheelchair a couple summers ago and said it was horrible. Some of the rides pre-date the ADA and have very steep ramps. I think she also found the ride operators to be unhelpful and things like that. Also, the disabled parking is on the other side of the road from the Park and while there is an overpass and it is ramped, the ramps are long and I think probably just barely compliant. Within the Park there are hills as it slowly in some spots and less slowly in others slopes from a hill down to the CT river. I can't remember from when I was last there, but I think most of the newer rides (and some of the older ones) have mainstreamed lines.
I have a friend who works at the Park so I'm going to try to go with her on one of her off days so that someone who works there can see first hand what it's like to experience the place in a wheelchair. I can stand to transfer and walk a little so I probably won't come back with the same experience of someone who can't stand, but with how my health fluctuates, I may not be that good when I actually go.
One thing to keep in mind as well is that Six Flags New England is, in many ways, a smaller amusement park that's kinda grown out of its limits in an odd way. It's an interesting mix of old time amusement parks and a modern larger one.