As someone who has gained, and so experienced both sides, its not that far from how the stories make out.
In general, its not that big a difference, at least for me, I don't feel fat all the time, only if I see my reflection or feel my body.
However, things happen which reinforce the idea that i'm not as thin as I used to be. I've always been a hyperactive person, so now when I jump around and stuff, it tires me out really easily and I can hardly run much, although I suspect this has more to do with being unfit then being fat, but you deffinetly feel the excess weight moving around.
As a gainer though, it is incredibly sexually satisfying to feel my thick flesh and bunched up rolls. My large belly, my squished thighs, it is all an incredible turn on. I'm not sure if its the same for people who are naturally fat, but I hope so, cause being fat is alot of fun.