Yes, Conquest has good sources. He wrote
Harvest of Sorrow before the Soviet archives were opened up, and his research then has been totally vindicated by the flood of information since.
I think the first version of
The Great Terror was also written before 1991, and then more data that had become available since 1991 was added to the updated version. I don't think that anything that was in the first version was invalidated.
Conquest was called many foul names for his anti-Soviet books during the Cold War, but he has largely been proved correct.
http://en.wikipedia.org/wiki/Robert_Conquest#The_Great_Terror
After the opening up of the Soviet archives in 1991, detailed information was released that Conquest argued supported his conclusions. When Conquest's publisher asked him to expand and revise The Great Terror, Conquest is famously said to have suggested the new version of the book be titled
I Told You So, You Fucking Fools.[6] In fact, the mock title was jokingly proposed by Conquest's old friend, Kingsley Amis. The new version was published in 1990 as The Great Terror: A Reassessment (ISBN 0-19-507132-8).
Some 'revisionist' critics have argued that examination of archives following the USSR's collapse in 1991 challenge many of Conquest's statements.[7] Although some aspects of his work continue to be disputed by those on the left, he is often regarded as having been vindicated by history.
Michael Ignatieff wrote "One of the few unalloyed pleasures of old age is living long enough to see yourself vindicated. Robert Conquest is currently enjoying this pleasure."[9]