EDTA is the chelating agent in csmb.
6.5 is the PH threshold where EDTA begins to detach from Fe only. To the other micros, it stays bound at much higher PHs.
How good is it? if your PH never gets much above 6.5 its pretty good. If you PH is higher than 6.5, you will most likely need an additional source of Fe (depending on your set-up obviously, low-tech vs high tech, etc)
A good additional source would be DTPA chelated Fe, which stays bound up into the high 7s, or Fe gluconate, which isnt much of a chelate so it doesnt stick around as long, but it is more easily available to plants than chelated forms.