When trying to figure out whether CDR is valuable or not, consider the following:
a) Do I have a skill that is short enough cooldown to be used multiple times in a fight? Reducing a bunch of 15 CD skills to 9 just isn't super helpful as they tend to be once-per-fight skills anyways.
b) Am I a burst caster or sustained damage? A burst champ would rather boost their damage they can drop at once, vs trying to boost sustained dmg thru CDR.
c) Note that the more CDR you have, the benefit INCREASES PER POINT. 10% CDR = 11% more spamming per unit time period. 40% CDR = 67% more spam per time period.
d) A tougher champ benefits more from CDR since they are more likely to live long enough to use their skills more often.
e) An autoattacker usually avoids cooldown reduction as it just isnt' as helpful. Their skills are usually longer cooldown once per battle ones.
f) Looking at Scaling ratios is helpful too. For example a Soroka would need like 7000+gold worth of AP to increase her Starfall by 67%. But 40% CDR which costs around 1600-2000g worth increases her Starfall DPS by 67% too! Due to lower AP ratio and relatively high base damage, a spammable skill like this is more helped by CDR.
g) If the champ has an Ult that is really key with a long CD, some CDR can be really beneficial. Malphite for example, or Malzahar. Having Ult available at every fight is really important! Even though Mal doesnt super benefit from CDR for his other spells, the CDR is tempting just for his ult availability.
You really have to look at CDR on a PER-CHAMP-BASIS, but hopefully these general tips are helpful.