yes of course you will feel like shit at first! you guysa re all addicted to carbohydrates! you have to make your body convert its metabolism over to a lipids utilizing machine.
fats are a wonderful source of energy, and you definitely can workout with a whole lot of intensity once you have made the metabolic switch. you will be much more stable throughout the day, and you wont experience any highs or lows. its a very comfortable feeling, functioning off of fat.
the only argument that holds any weight against a zero carb diet is that carbs= insulin, and insulin= storage of protein in the muscle at an accelrated rate. so the argument goes...if you dont have carbs, you arent storing protein as quickly as someone who has carbs is. but this is false! proteins and lipids BOTH secret insulin, just in much smaller amounts than carbs...but yet, its a sufficient amount to drive proteins into the muscle tissue. and yet, its NOT large enough amount of insluin to lower GH levels, or testosterone levels, or catechamolen(sp?), camp, or lipolysis levels.
of course, if you are a "hard gainer" you could always try a targeted ketogenic diet, where youc onsume about 20 grams of fast acting carbs just prior to working out, and this will give you a big boost in insulin levels, and give your body the ability to replenish muscle glycogen levels post-workout without any additional carbohydrate intake.
BUT, you dont HAVE to eat carbs to get this same insulin/anabolic efffect around your workout. the bcaa leucine is an insulin stimulator, as is glutamine. taking these pre, during and post workout will boost insulin, give your body readily available aminos(so you dont break down any muscle tissue), and help restore post exercise gh levels.