In the 8 or 9 months that I've been eating low-carb, I've experimented a lot to see what works best. One obvious thing I've varied is the amount of carbs I eat, making sure to always keep it between 40 and 60 g per day or less. I don't think it's even gone far above 40, aside from the handful of weekends that I've allowed myself to indulge.
Recently I tried out a more teetotaling approach, probably taking in around 10 g per day, 20 g at most. I ate a good amount of food, just almost all protein and fat. Your body and brain do require some glucose, but most of this can be made by eating protein, as the liver will convert the amino acids into glucose for you in a process called gluconeogenesis.
Still, I felt less energetic on this stricter regime, so I thought Thanksgiving weekend would be a good time to change things up. I had a good deal of carbs over the weekend, though nothing crazy -- some not so saccharine cherry pie with a little whipped cream, and the odd handful of candied walnuts or honey-sesame covered cashews. As of yesterday I've stopped eating sweets, but I've stepped up the carb count to around 30 - 40 g per day for the first time in awhile, and I feel much more invigorated than before, like I did back in March or April when I also ate slightly more carbs than I had been recently. The clearest signal is always libido -- impossible to misread that gauge. I was far from ignoring girls recently, but now I'm back to teenage levels again.
Now, 30 - 40 g is still pretty low and doesn't include candy bars or other junk -- a bit of roasted red pepper, half a fresh pear or apple, some peanut butter, maybe a small corn tortilla too.
Some people who've tried low-carb have told me they felt somewhat fatigued or foggy on it even after the week-long or so adjustment phase. There could be all sorts of reasons for that (too much protein and not enough fat, for example), but one may simply be that they were going too far in the low-carb direction. If you haven't tried it out, or if you have but are questioning how great it is, try dialing it up in little steps until you notice a big change. Although humans in general are designed for low-carb diets, there's still variation among individuals. You may just need a bit more carbs than others. The Eskimos and Masai seem to thrive on incredibly low levels, but you may need something in the 50 - 60 g range. The only way to find out is to dive in and play around until you figure it out.