Good replies
Low carb can be beneficial, especially if your carb consumption is currently greater than the recommended 285g (roughly). By low carb I don't mean Atkins 20g, I mean a more healthy 150g. Let's take the average regular "healthy" diet with a little treat:
Breakfast - bowl of cereal with a banana and glass of juice
Morning snack - a piece of fruit
Lunch - pasta salad
Afternoon snack - donuts that the boss brought in (it would be impolite to say no after all!)
Dinner - mashed potato, veggies and meat, with nice uncut bread on the side
After dinner snack - fruits
A healthy diet with a little cheat. The total carbs in that, depending on portion size, can and often is far greater than the recommended 285g. My house mate lives on breads and pastas, his carb consumption is often quite high, luckily for him he's not over weight at all and is perfectly healthy!
Now imagine how much carbohydrate is in the average junk food diet, pizza, chips, lots of bread products, take aways, not to mention pretty hefty portion sizes. A person eating like this is consuming their body weight in carbs every day! (slight exaggeration, but you get me

). And so they start to suffer the negative effects of too much carbohydrate, and too large portion sizes - including stubborn weight gain, fatigue, brain fog, aches and pains, headaches, emotional imbalances and so on.
If everyone was to cut down to even the recommended carb intake instead of crashing on extreme low carb diets, they'd see a big difference in their health and weight loss, especially in the long run.
Now for zero carbs. I actually don't believe this is possible. People who claim to be on a zero carb diet are actually on around 20g a day. It's not possible to survive on a zero carb diet. No fruit, no veggies, no flour or bread products, no rice, no dairy products, just meat!
EDIT I do actually know someone who is eating just meat and so zero carbs. But this is a body builder on an experiment for specific results, it's not meant aa long term healthy at all (just like the 20g on Atkins isn't meant to be permanent).