I do think that Rawls' hypothetical original position is a reasonably good basis for conducting the thought experiment.
By hypothetically placing all agents in a completely equal position behind a "veil of ignorance", we can agree on principles that are reasonably fair. I believe that fairness is fundamental to any conception of justice.
Rawls chooses equal basic liberties for all and social and economic equality as the two principles which would emerge from this hypothetical contract. I'm not sure I agree with the second principle because there is a risk that an entitlement culture embeds itself in society and this ultimately leads to another form of injustice.
I think there should be protection for the genuinely disabled and destitute but, for example, I don't think it is fair that 70% of my income is taken away from me by force to give it to people who cannot be bothered to work for a living (economic equality). I also don't think that social equality is always a good thing if it means that it holds back the genuinely talented members of society by dumbing everything down to the lowest common denominator.