Spell Speed works really well for BLMs because they have no auto-attack. In terms of just pure ability damage, Speed beats all other 2ndary stats (except acc), hands down. Even with melees who do have auto-attack, Speed still wins with the possible exception of Determination. That being said, 2ndary stats hardly makes a difference; if you are having significant DPS issues, its not your 2ndary stats that's the problem.

The absolute raise in crit is static, you will always require the same amount of crit points to get a 1% increases. However, a percentage raise in crit is worth less the more overall crit percentage you have.

Speed however works differently. The lower your GCD is, the more Speed points it will require to lower it by a specific absolute amount. However, the lower your GCD is, the more an absolute reduction becomes. -0.1 sec is worth much more at a 2.0 sec GCD than it does at a 2.5 GCD sec. A lot of people seem to be confused on this, especially Monks since Greased Lightning lowers their GCD by so much already that the the reduction from Speed seems even more insignificant that what 2ndary stats already are.