I for one am glad to see some diversity of opinion in this thread.
If I were a pragmatist in the CIA, concerned purely with US strategic interests, I would probably argue even from that narrow perspective that the US blockade has helped more than hindered Castro.
By isolating his regime the blockade has actually insulated Cuba against the normal changes one might have expected through the sheer gravitational pull of a superior power up against a weaker neighbour.
And it has also handed him a propaganda victory. To ban US citizens from visiting Cuba, in the name of liberty, doesn't look to good from the outside.