My lone experience with supersonic flight in 2002 was in the backseat of an F-16. Now move forward to the release of the Iris F-16 and that was the first time I could say that the take off and climb brought back great memories. I told the pilot afterwards that my immediate impression when he hit AB was that it was like a slingshot because until we hit the climb it felt like my mind and my senses were trying desperately to catch up with my body. My instinct told me to grab hold of something so I wouldn't be pulled out of the cockpit. . . .very weird sensation. The sensation in the first take off with the Iris F-16 was something similar. . .the sensation of instant speed was there. After that though JimJam is correct in that in FSX at least I don't get that feeling that my best option is to just look straight ahead because I wouldn't be able to focus on anything outside that tunnel anyway. Using TrackIR heightens that feeling somewhat, but FSX is a long way off from the real thing in that sense.
One other thing that I have always felt is way off also, and I only get this feeling in the fighters, is the percieved height as you sit in the cockpit. I have always gotten the feeling, no matter what airplane it is, that I'm in a go-kart comparatively speaking. I don't know of any fighter, freeware or payware that gives you the true sensation of sitting "up" in the cockpit. When the engines are running, the canopy is closed and pressurized and the crew chief and pilot are running all the pre-taxi checks, he looks down at the crew chief. That's not what you feel in any Sim Fighter. I get the feeling that I could look eye to eye with him. That's something it would be great to have corrected at some point.