I think DL gear is a little bit overestimated. Most (I'm saying it right now before you blast me: not all of them) of those pieces are just rare timesinking gear ppl wear just to appear cooler.
Of course a few of those DL pieces are kida unique and useful, and difficult to replace with more "casual" (aka gc/materia/AF) gear, like HDL gloves with +45 enmity that is a rare stat on gloves (only militia set has +10 enmity on the same part, and they're not so much easier to get). But the truth is that most of them are useless (mage gear sucks, really, do you want to talk about Darklight Cowl?) or can be replaced greatly with GC combos (expecially with most of the new GC weapons we are going to get in 1.22c, that will require 4/5+ gc equips at the same time to activate their uberness), AF or melded gear (you can have with much less effort).
Monk is a nice example to explain it better.
Monk
Darklight Corselet
(the only DL equip on Monk that would have to be replaced by GC piece to reach 5 pieces bonus)
Requires: MNK DRG BRD
Defense 153
Strength+20
Accuracy+18
Evasion-7
Attack Power+10
Critical Hit Attack Power+30
Store TP+40
vs Storm GC gear
Storm Sergeant's Tabard
Suits: All Classes
Defense 158
HP+60
Strength+10
Accuracy+5
Attack Power+10
Storm gear (3 or more pieces): Enmity-20
+ only combo bonus stats from other GC pieces:
Earrings: Critical Hit Attack Power+7 (requires 4)
Hose: Accuracy+10 (require 3, so earrings and bracelets, but earrings require body too)
Bracelets: Strength+7 (like Hose)
Storm Lieutenant's Hooks (1.22c):
Damage 133 Delay 3.3 DPS 40.30 Blunt 100%
Accuracy+20 Critical Hit Rating+5
Storm gear (5 or more pieces): Attack Power+30
(Very similar to Garuda's Talons, but with +30 Attack Power bonus)
This gives the body (actually the 5th piece of the combo) those virtual total stats:
Defense 158
HP+60
Strength+17
Accuracy+15
Attack Power+40
Enmity-20
Critical Hit Attack Power+7
So far:
5 more Defense,
3 less Strength
3 less Accuracy
30 more Attack Power
23 less CH Attack Power
7 more Evasion
Different stats
Enmity -20 on GC set
Store TP+40 on Darklight
Something that with a blue name, and cooler appearance would be exhibited way more then a darklight piece.
And, to be honest, you would never use those Storm Lieutenant's Hooks over a primal weapon if you don't have 5 pieces to activate the last stat, so I should have added more stats (like CH rating), but whatever, let's stick to the bonus only.
However this is just a full GC gear versus 1 DL piece. Things are a little bit more complicated if you have a full DL set. But I really believe that a combination of GC, materia and AF is better then a full DL.
Going on as Monk:
Darklight Eyepatch is the best DL head piece for Monk, but is not really too far from AF, that also has 35 more Evasion, 3 more Strenght and 5 more Intelligence, traded for +15 Attack Power, and a huge boost on CH Rate and Power. And this can be stacked on GC gear too, so it's pretty good afterall. But there are also other options, like Silver Tricorne with +30 Int meld, Sipahi Turban with +7 Str and -7 Enmity, or the old Paragon's Crown, with useful Strength+10 Vitality+10 Dexterity+10 Intelligence+10. They are totally not worse then Darklight Eyepatch, it's just solid damage VS crit rate.
Darklight Bracers are good for the Accuracy bonus, but have again Evasion malus and only Attack Power, that is good for AA damage, but is not as good as a pair of Felt Bracers with Strenght double meld, since DPS is more based on WS (and they also have a little bit of Accuracy).
Darklight Caligae is nice, but they only have +10 Accuracy that can really be useful on Monk (not a big deal since you can get about 50+ with the GC gear I've mentioned before). Explorer's Moccasins have Attack Power+5 and Critical Hit Rating+30 that is probably as good and you can buy them from market at a low price. And Temple Boots are not so bad.
Dragoon
Darklight Helm is actually pretty good. One of the most useful DL pieces. However it's not too far from Paragon's Crown or AF (that has lower "sure" stats, but has +40 CH Power).
Darklight Cuirass has huge Def (compared to AF) and CH Power (almost the same that DL set loses from helm), but has no TP bonus. Attack stats are similar (DL is a little bit better), so it's Invigorate buff vs CH power. Worth testing at least (but I'd choose AF).
Better then GC gear? No.
Darklight Gauntlets have very nice stats. Should test if double melded STR Felt Bracers are better on bosses.
Darklight Sollerets are very good. Power vs -Enmity and Elusive Jump buff (so even less Enmity). I prefer DL this time however. Also no need to give up on useful GC pieces.
Darklight Caligae is probably the best on DRG, since it's hard to find Piety on feet.
PLD/WAR (HDL)
Heavy Darklight Gauntlets are the best gloves for tank, so not a big deal.
Heavy Darklight Armor can't equip head piece, but has huge stats and enmity bonus. WAR and PLD AF head+body are also good, but not as good. You get more Def and good stats from GC Officer gear however, but it requires Sanction.
Heavy Darklight Flanchard seems nice, but AF trousers + Sentinel's Sabatons are better.
About DD War, things are a little bit different however.
WHM/THM
Not worth talking about it. Cowl really sucks for both classes. the only useful piece is Darklight Gloves for WHM, but AF is good too. Doesn't really make the difference.
BARD
Bard's situation is similar to Monk. Can equip almost the same pieces from Storm GC, except trousers, but has +10 Accuracy +10 Dexterity Storm Sergeant's Jackboots and the new Bow (1.22c), with double melded DEX gloves.
About skill bonuses from AF, can just macro them to rapidly switch gear before the skill, then switch back.
Darklight Caligae is nice, but Storm Lieutenant's Bow requires 5 GC pieces.
I'm not saying that DL gear sucks, of course (well, mage DL sucks, really), but just that most of it is overrated and it's not worth spending days trying to get it (unless you have fun doing it), since it's not the best gear in the game. Not most of them at least.
So far I believe that only a few DL pieces are really useful and worth:
Heavy Darklight Gauntlets: best gloves for PLD and tank WAR
Darklight Helm: good, maybe better then WAR AF (don't know how much the Antagonize buff is worth), but I'm not sure I can say the same for PLD and DRG
Darklight Sollerets/Darklight Caligae: rare to have those stats on feet pieces, good for DRG
Heavy Darklight Armor can't equip head piece, but has huge stats and enmity bonus.
Darklight Gloves: best gloves for WHM, but AF/GC is good too
5-6/15 pieces.
The other pieces are not good, or at least nice but not worth compared to gear you can have with much less effort.
That's just my opinion, discuss.
And forgive me if I made too many errors writing this post. English is not my native language.