Some genaral guidelines:
Getting INTO ketosis is MUCH more uncomfortable than BEING there.
Like the guy said above, once you are IN ketosis, clearly THERE, and once your body adapts, you will not feel AS bad. You will not have a lot of power for workouts, but that mentally "sluggish" feeling will go away. Also, if you are used to ups and downs in energy throughout the day (as your blood sugar goes up and down, as insulin goes up and down, etc). then you may find that being FULLY in ketosis actually gives you a more "balanced" and "even" feeling (less ups and downs).
But again, in terms of POWER for your muscles it will NOT be there (hence the reasoning behind modified keto diets such as the CKD and TKD).
The reason I emphasize the aspect of FULLY being in ketosis is I think that a hell a lot of people who say they have "tried" to do a keto diet have never actually fully gotten into ketosis and experienced it. They piss and moan and whine about how much they crave carbs. Once you are REALLY in ketosis, those cravings stop (or at least diminish a lot).
BTW, going ino ketosis the first is always much more difficult than each subsequent time. The first time I went into ketopsis, it took like a full week at less than 20 G/carbs a day. I was ready to like kill someone for a Pepsi or an Apple. It took a FULL WEEK.
But that was the first time. After I got used to it, and while I was doing a CKD (which has you go into and out of ketosis each week) I could get into ketosis in 36 hours. The last 8 hours I was a real bear (grrrrrrrr) but once you "switch" over - its a lot easier.