Giving my own opinions, not necessarily the best.
Does it still do damage? If yes its good, if not its not really worth it.
Enlightened spirit (Warlock PrC) has a greater essence that gives much better effects in the latter case. If the damage is still there, its a good if situational blast
The scaling is my biggest problem here. You basically get an epic item effect at level 14. On the other hand you lose an invocation slot for a stat increase that is less important than others (warlocks have little trouble to hit and shouldn't get hit that often anyway). I'd probably still take it if I was a melee warlock.
The Bell Tolls For Thee
I am honestly unsure about this one. It helps kill enemies while keeping yourself kind of healthy. It could be fun if you want to play a desperate suicide bomber who can't comprehend why he just won't die. Also you should use the old specification that you need a conscore for this otherwise this is entirely too abusable for undead warlocks (they would be basically immortal)
Sensible and very useful in the beginning. The enemy is dead, they just don't know it yet. Though the original summon swarm invocation can do somewhat the same to more people in the beginning.
Absolutely appropriate. A very nice choice for a spell-based invocation that fits the warlock.
So you get a deflection bonus that disappears if you get an armor bonus. Only really useful if you really pump Charisma I think, though if you do this is a very good defensive option. I assume it lasts the usual 24h?
Enslave the Planes
Good if your campaign centers around planar travel but otherwise not that great.
Very nice. Helpful to others but at the same time giving you a switch that ensures you can still affect them if they willingly turn on you.
Probably appropriate. I have little experience with the line of spells. I also get the feeling you do a lot of suprises and traveling across planes in your games based on these invocations.
Same problem as with the dexterity one. Good for Melee Warlocks I guess.
Ouch, this one is in itself great for DC upping. Same problem though as invocation slots are rare.
This one is creepy and sick. I whole-heartedly approve. Though at this point the payoff is a bit weak for a Dark invocation. I'd go with a bonus equal to 1/3rd the amount of HD you harbor in souls (or character level if it applies)
Creepy. Could be fun though.
Pull the Strings
Oh, ouch. Bela Lugosi makes everyone his bitch.
Depending on what you play this can be entirely too strong. Though it has a great feel to it.
Servant of the Damned
So a quasit now as a mediocre investment and according to the malconvoker guide the best combat summon monster VIII at level 17. This is one of those invocations that you probably at least train into later. Though it becomes useless once you hit epic epic possibly get Shadowmaster. Maybe add something inbetween at level 12 to keep people interested in it over time.
Steed of the Damned
Not sure if it is worth a greater invocation. The 19th level kicker is great but I'm not sure wether it makes much difference at that level.
Does it stack (temp HP and damage). If not its a nice starter for delivering damage at lower levels. which is probably what it is meant to do.
see my comments on the others.[/QUOTE]