I have it all turned off. I find it off-putting. Maybe it'd be ok if the effect was only very small, I don't know, but I like the game without it.
I have mine set up (so it is quite a subtle change between 45mph and 140mph
I have field of view and movement set as follows (I use helmet view)
Field of view
Bumper 68
Hood 70
Roof 73
Chase 65
Cockpit 68
Helmet 68
Rear passenger 90
Field of view speed sensitivity Yes
Min speed sensitivity 95
Max speed sensitivity 115
Movement
High Speed Shake On
World Movement 100
G-Force Effect 100
Show Helmet Yes
Depth of field No
Helmet look to apex 55
Helmet leaning 100
Camera leaning 100
Here's a video with those settings.
The next video shows the effect of the sensitivities set as follows (extremes) in various views.
Min speed sensitivity 75 (minimum)
Max speed sensitivity 175 (maximum)
This last video shows the sensitivity settings reversed so that fov increases (widens) the slower I go). It's probably more useful than the example above if you want better visibility in corners. I'll stick with my normal settings from the first video though
Min speed sensitivity 125 (maximum)
Max speed sensitivity 75 (minimum)
I believe the sensitivity settings apply a percentage to the actual field of view for each camera.