Whenever my old irons no longer fit my game.
Played R-flex Spalding TopFlite Synchro-Dyned (1959?) from 1967-1974; needed switch to stiff shafts and smaller head.
Played MacGregor MT blades, stiff, from 1974 to 1994; switched to Ping Eye2 clones (stiff) for benefits of cavity-back and a custom fit.
Switched to Callaway X20s in 2009 as part of move from stiff to regular.
Switched to Callaway X20 Tours (PX 5.0 flighted) in January 2011 to get smaller head, better loft spread; may reshaft to lighter steel (around 100 grams), as 5.0 a bit heavy and halfway to stiff on frequency-matching scale. Have tested lighter shafts at demo days and expos, need actual launch-monitor session to pinpoint best shaft choice.