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I had a unique opprtunity a couple of years ago to do direct comparisions with a mix design using flyash, and one without flyash. The project involved a temporary precast prestressed concrete production facility for a shopping center parkade.
The major difference in the two mixes was the inconsistency of strength gain with time with the fly ash mix.  The curing conditions and mix condition were carefully controlled with curing temperatures and environment kept constant and warm. Testing for other properties indicated little or no difference between the mixes.  The design strength was 2500 psi @ 16 hrs and 6000 psi @ 28 days for beams, 2500 psi @ 16 hrs and 5000 psi @ 28 days for slabs.  After this experience I do not allow flyash in mixes where strength gain is critical, but will allow flyash for all other applications.