Well...it kinda depends on what you replace it with! And how messed up your diet is right now. All I'm advocating with this Primal/Paleo diet stuff is that you eat the kind of stuff humans ate for hundreds of thousands of years, before the advent of agriculture. So what you should really feel is just "normal" - the way your genes program you to be, not sick from eating stuff that is toxic to you.
Generally, I just feel smoothed out, hunger-wise. I usually feel like I could easily skip a meal or two, without feeling overly full or bloated like I would if I'd just eaten a bunch of fast food, say. Also, the energy levels are more consistent throughout the day. I never feel drowsy after eating a big meal, and I never have any kind of crash (other than hangovers, booze being my kryptonite). At the beginning, I think most people tend to overeat out of some feeling that they're not getting enough without the grains. After a while, your appetite really moderates and becomes more in tune with your activity levels. The best way to describe is: having a meal has very little effect on how I feel, regarding mood, energy, whatever. I feel the same at the end as I did at the start.
It also depends on how lean you are to begin with, I think. If you have weight to lose, and you go really cold turkey AND keep a close eye on the portions, you should lose the weight at a healthy rate. (I firmly believe weight loss should be gradual, and I aim more at "body recomposition" than weight loss anyway, i.e. increase muscle mass at the same time you cut fat)
