Well, Lindsey, I used to have shakes every day, and on some busy days I would substitute them for food.
The amount of fiber in them can make things........explosive.
By shakes do you mean juice? Or do you mean something pre-packaged with added supplements?
I never have, but a friend of mine does it every so often. He goes back to his regular diet, then gets a bug up his ass to try again. He likes to get gum to chew on to replace the urge to be chewing food, which is pretty intense sometimes when you're drinking all of your meals. Also a bit of lethargy in the first 3 or 4 days after you start. He swears his energy level goes up and his head is much clearer after that though, but that might be a placebo effect in my opinion. Who knows.
I've seen the same thing about the first few days being awful. Supposedly it is all the toxins leaving your body. Obviously there will be some digestive upset in pretty much everyone when you drastically change your diet in any way. So I'm sure that will be totally awesome.
Actually, I think it's the FOOD leaving your body. Seriously.
Having done it maybe between a half-dozen and a dozen times, my experience is that the first day isn't that hard, but "feels weird" because you keep having the automatic reflex to eat.
Then the second day, you can alternately feel energized and worn out. I think this is because (if you use the classic formula, anyway) you are getting energy from the sugar in the juice, but your digestive system basically not "running" any more uses less energy. The trick is to be very regular with the juice drinking and to predict when you're going to need energy, because the juice and the fat-burning is all you have, and if you're going to burn a lot of calories, you're going to need more juice--BEFORE your activity and during. Otherwise, you will have an energy crash, because your body only mines fat so quickly.
After three or four days, I start to miss food. Interestingly, I think this is when you really start losing fat in earnest. Might be interesting chewing gum. Never tried that. I usually crave some salt, too, none of which is in the juice. If found that giving in to the salt craving, say, by drinking beef or chicken bouillon, or something with no solids and very lo cal, the salt apparently enhances the appetite, making things worse. I think exercise actually helps, as what you crave then is the sugar in your juice, and if you drink the juice you need, it tends to satisfy. I think my most successful fasts took place when I was on a good exercise routine and stuck with it (naturally, you need to drink juice constantly while doing cardio, and stop if you have a sugar crash.)
The exercise, I think gets me from day four to day fourteen. Also, though, I have Degenerative Disc Disease, which complicates everything for me. If I get a flair-up (which comes more frequently than they used to), eating, for some reason, helps. I also found out I was caffeine addicted the first few times I fasted. This was easy to correct for when I knew it--the key was to do one of two things: ramp down on the caffeine before fasting, or alternate between juice and tea while fasting, or some combination.
If there's anything to stress, I'd say it's the exercise. Of course, the benefit is twofold: you crave the juice you need when you exercise, and the exercise helps you burn fat and remove toxins. When I put in the best effort of exercising, I never actually felt the urge to stop the fast--and that's the real struggle, in my book. You can overcome the crashes by learning from them, but what makes it "hard" is the urge to cheat or quit.
By "classic formula" I mean diluted maple syrup, lemon juice and cayenne pepper.