You are right, for the first week or two, your do feel like crap. As your body switches to using ketone bodies as energy, you go through a real down phase. However when you come out, you feel great.

Anecdotally (I hate anecdotes about health, but argh) I used to need to eat constantly to keep my energy up. I'd have breakfast, and by the time lunch came around I had to eat, or face passing out. These days I can skip breakfast (or just have a coffee with lots of cream) and that will keep my going until 2-3pm where I will have a lunch. You have no idea how liberating it is not to always be thinking about food.