I just recently changed my diet (almost a new years resolution) and the change has been to the magnificent Paleo diet. My diet prior to this was the "I work out, I eat anything I what when I'm hungry." I actually never really felt the significance of a good diet, and never needed to. Since I'm still a teenager, my youthful metabolism had allows saved my rear. Other than having a nagging sweet tooth, I've been happy with what I eat. Oh, and the benefits are super nice also.
I only really have been on the Paleo diet for little over a month, but that contributes the almost immediate visible side effects. I had previously weighed over 155lbs and now I weigh less than 150lbs every time I check. I have noticed a wonderful increase in energy and overall strength. I figure that the weight loss is fat since I can now see veins in places where they weren't before, and progression is a bit more... Progressive.
Ultimately, its been pretty good lately. But I am getting some bad protein farts, and I pretty much battle my sweet tooth every night. The change itself is a bit challenging since I live in house that eats rice in every meal.
Oh, and some nights I'm hungry every second even after I eat. Is that because of the lack of starch or whatever? I haven't really looked into it.