Critics definitely consider the personal benefit. The problem is that they don't trust that it'll actually be there, or that it's a system that actually can work as it has been designed.
For example, in the 80 years that it's been in effect, the OASDI tax rate has increased sixfold and then some. That increase was fairly steady for a period of time - and the reason it's stagnated recently is not because the tax revenue is sufficient (it isn't) but because political ramifications have prevented the tax rates from increasing. Obviously this isn't sustainable. You can say, "well, the solution is just to raise taxes" - but relying on periodic changes to tax rates is not a recipe for creating a system that can achieve a stable steady state without being subject to political meddling in the long-run.
A further problem is that Social Security hinges upon the idea that the current working population will be responsible for paying for the current retiring population. Unfortunately, the respective sizes of these populations can vary dramatically, which means that some generations will end up paying far more than others. People talk about Social Security like it's true time-shifted income (ie, savings), but it's not.
One way we've dealt with this so far is to increase the number of people who are paying into the system while remaining ineligible for collecting benefits from it, but that's also not sustainable (and it's also a really horrible way to treat people).