About two years now, and I'd been doing it less strictly / on-and-off for a year and a half before that.

I've definitely noticed feeling much more focused and productive since starting, and it also helped with dropping from 210 to a bit over 150 pounds in the first few months of 2014.

Also anecdotal, my cofounder at my current startup Cyph has been on keto for close to a year now, and in that time has had a heart condition almost entirely vanish and his body fat drop to something like 6%.

+1 for keto here.

the only real downside is that it's extremely difficult to maintain the discipline to avoid carbs! also the social aspects.

I would actually say that it's extremely easy, but only because any carb cravings I may have are met with the flour/sugar substitutes and pre-baked goods from LC Foods (holdthecarbs.com), along with other common carb replacements like shirataki noodles. It's pretty nice that I can freely eat awesome dishes like pizza with the crust fried in clarified butter that people who aren't on a high-fat diet would never touch.

That said, it would certainly take some discipline without something like LC Foods, and I suppose there's also the issue that keto kinda forces you to become an amateur chef (but I at least enjoy that aspect and don't find that it's super time-consuming).

While the social aspect is of course an issue, one interesting thing socially is that anything I cook for guests at home is LCHF, and people are always shocked when I mention it in passing later on; e.g. one reaction to my stevia ice cream was something like "I thought it was weird that I wasn't getting a headache like I usually do after eating sugar!".

yeah, sure, all that stuff. but it's still hard to not order fries when i go out to eat.

the reason i got fat in the first place is because i love, love, love, love, love carbs. real honest to god carbs, not half-assed replacement like fake noodles.

Ah, then maybe it's just easier for me because I've never really liked fries and have always seen noodles as nothing more than vessels for sauces/cheeses/meats/etc. (a common complaint about shirataki is that it's flavourless, but to me good quality shirataki is indistinguishable from ramen noodles — probably because I'd never noticed any particular flavour of noodles to begin with).