Keto is much more restrictive than Atkins but also more focused on long term benefits rather than just weight loss. Basically, you're on a very strict low carbohydrate, moderate protein regimen in which you want your body to enter a state of ketosis where it can no longer rely on sugars for its source of fuel. It takes a few days to enter a state of ketosis (and you'll feel like crap the first few times you do) but once in, your body will burn fats for energy. You can check to see if you're in ketosis with urine sticks or a breathalyser. Because the diet is high in fat, you'll generally never feel hungry and your body will use your own fat stores as a fuel source. My average daily caloric intake is about 1900 now and I often have to force myself to eat. The main drawbacks are that there is simply no cheating allowed at all. If you eat too many carbs/sugars, you'll drop out of ketosis and back into using sugars for energy and you'll have to start the process over again. It can be difficult to follow if your lifestyle involves a lot of socializing/eating out. It took some adjustment for me to get used to eating like this but it's mostly second nature now. I do sometimes get complacent and stop doing detailed tracking of my food intake and I find myself out of ketosis because I ate too much protein (too much protein will also drop you out of ketosis) but generally I can stick with it.

Krypto, I wish it were muscle gain. I've lost a lot of muscle mass over the last 5 years. I used to hit the gym 3-5 days/week and lift moderate/heavy but I had a health scare and was told to stop heavy lifting. I can only bench about 40% of what I was doing 5 years ago.