My up close experience mostly came when I went to several Jets/Pats games earlier in the century at the old Giants Stadium. Even then tickets were readily available in the Parking lot. I never paid more than face value of any ticket, and a couple of times we got in for 20% below face. Fans were what I would call "respectfully hostile". They gave us an appropriate amount of crap given I was wearing Pats gear. Once seated it was more of the same, but abated as the game wore on, mostly because I wasn't obnoxious and just rooted FOR my team, and didn't sh!t on their's.
Online, I was, much like Jetfan 79, a fairly regular participant on the GangGreen site before it partnered with some other site and pretty much disappeared. I enjoyed it and for the most part I was accepted by most in the role of the somewhat annoying opponent's fan who posted about Pats topics and during the 2 Pats weeks a year. Got into some good football talk most of the time, and was summarily dismissed by a few on occasion.
Personally I like when fans of other teams come to discuss football when their teams are playing the Pats. I wish more did and we were, as a group more welcoming. The site would benefit from differing perspectives they would bring. But that just my opinion.