I'm not saying one should never work at a startup. I'm just saying that startup offers shouldn't be judged any differently from any other job offer. There might be smart people at startup X, but there are plenty of smart people at Google or Facebook or what have you as well. Take the expected value of your options and your salary and compare it to any other offer. No special rules should ever be made for startups except in extremely rare circumstances. Investor connections that one might get from being an employee at a startup are often vastly overstated as well. If you want to be a founder yourself, you need to just go found something--working for another founder just isn't the optimal way to get there, especially with the extra time commitment that many startups involve, meaning you won't have time for your own side projects.
One of those rare exceptions I mentioned might be if you are asked to come on in a CTO or VP Engineering role for a new and emerging startup. This sort of experience can help you get executive positions later on at larger and more stable companies if that's the career path you want.
Edit: I want to clarify my final point a bit more. Titles matter a lot in early-stage startups. Almost by definition, VC-backed startups are going to undergo rapid expansion--and that means lots of hiring. And that means that unless you have a senior title from the beginning you face the real risk of simply having people hired over you rather than being promoted yourself. This is especially true if you are hired into a junior role in the beginning. All of this means you may not get your desired leadership role in the end at all if you're not careful.