The recent exploits of the voidwalker have garnered him some attention as of late, but the day-to-day uses of this minion tend to be less glamorous. For Warlocks who haven't spent points improving him -- and most agree that it's generally a poor idea to do so -- the voidwalker is only useful now and again when attempting to solo an elite. For any content that is intended to be soloed, there are faster and more efficient methods; and most group content is beyond the voidwalker's ability to be be helpful in without, well, a group. I've actually had a great deal of time to compare the voidwalker to Hunter tanking pets, since my girlfriend plays one. Frequent changes have made this comparison rather variable, but what it boils down to is that the voidwalker has a great deal of damage mitigating ability compared to a hunter pet, but the Warlock's inability to Feign Death, and the somewhat high threat generation of Health Funnel makes it a very precarious balance to avoid pulling aggro off of the voidwalker even when Health Funnel is the only spell being cast.
I think Ghostcrawler summed up the succubus best when he mentioned that out of all the minions, she's doing the worst job of pulling her weight. Considering she can't weigh more than 90 pounds, this is a pretty serious issue. The main draw of the succubus is the crowd control she lends to the class. I remember dozens of times back in Shadow Labs when my Mage-less guild needed me to seduce one mob and Banish another in some of the more difficult pulls. Problem is, seduction is insanely difficult to wield effectively, and while three points in Improved Succubus can make it a little less unwieldy, most Warlocks avoid that otherwise useless talent altogether. Those three points are much better spent elsewhere, particularly considering how very infrequently Seduction is actually needed in a modern instance, and how very unreliable it is compared to other forms of crowd control. I sincerely hope that the talent will eventually be removed and, its affects integrated into the base skill -- much like what they're doing with Curse of Elements and Enslave Demon.
The felhunter is my minion of choice as an affliction Warlock, though that is a somewhat controversial choice. Many people prefer the imp as a dark pact battery, because it has faster mana regeneration, can Phase Shift, and Blood Pact allows for more life-tapping. And while these are all good reasons, I feel the benefits of using the felhunter as a mana battery outweigh them. The felhunter's mana pool regenerates slower, but it's also larger than the imp's, so you can Dark Pact several times before needing to wait, instead of just once or twice. As for phase shifting, I've used my fel puppy almost exclusively since level 75, and have yet to see him pull something I didn't want him to pull. And best of all, the buff he provides turns into 720 mana, 20 spell power, and 0.29% crit chance. Not a bad tradeoff for an HP buff that works out to about one-third of a life tap. I should add, however, that I don't feel Improved Felhunter is worth the two points. The improvements to Fel Intelligence are neglidgable, and the faster mana regeneration only works if the felhunter is attacking at melee range, which very quickly results in a dead minion on most bosses, even with the recent HP boost.
Even if they're not my favorite affliction minion anymore, imps still have an important role to play. With the numerous buffs available in talents and glyphs, and the passive bonuses to fire spells attainable through Master Demonologist, the imp is like a miniature Godzilla in destructive ability. And, with the ever growing population of Warlocks who are dividing their talent points almost evenly between demonology and destruction with the intent of maximizing their fire damage, we're sure to see plenty of them around. I've seen reports which show an imp's contribution as a whopping 20% of a Warlock's overall damage. And Blood Pact serves as a nice little bonus on the side that the whole raid can enjoy!
I'm not going to try and pretend I can speak about the felguard with any sort of real authority. The only time any one of my Warlocks has ever even had the ability to summon him was on the PTR for patch 2.3 when I speced demonology on a lark. Someday soon I really must spend the 100 gold to spec demo for a day, so I can get some real experience with this minion. All I can say with certainty for now is that a lot of people love them for leveling, a lot of people love them for PVP, and too many people bring them to raids, only to perform abysmally on the DPS meter.
Infernals are actually quite a bit more useful these days than they were in days of yor. With their massive amount of HP, quick summon time, and high damage output, I've occasionally used them to tank the last few seconds of a boss fight after the tank was slain. Even if the tank doesn't die, the infernal's massive DPS can be handy for affliction or destruction Warlocks looking to eek out some extra damage in the last seconds of an intense boss fight.
In light of the improvements made to the infernal, I took my doomguard out for a spin, and ended up with a somewhat conflicted view. Once I had one enslaved, the doomguard was absolutely unstoppable. I was literally sending him at groups of three or four of level 78 mobs and watching him solo them with ease in under thirty seconds. As a tanking or soloing minion he would be unmatched...if he would just stop turning all that unstoppable power against me. Even with Curse of Elements on him, to make him weaker against my Enslave Demon spell, I had a pretty rough time keeping him under control for more than a minute. Hopefully the upcoming changes to Enslave Demon will deal with that problem. Unfortunately, even if the doomguard is transformed into the model of minionly servitude, the rather arduous methods required to summon one mean that he won't see much action outside of soloing. Update: As numerous comenters have pointed out, I made an error in saying that the doomguard is difficult to maintain control of. I was using Curse of Doom to summon this minion, and having a great deal of difficulty holding onto it after enslaving it. I didn't think to experiment using Ritual of Doom (which I have since taken the time to do,) which provides all that doomguard bad-assery, with none of the pesky 'turning on its poor clothie master and eating him' issues. Of course, the summon still has a very limited duration, but the doomguard can be dismissed without setting it free on a group of unsuspecting party members. Now the only remaining issue is convincing four other people to help you summon the thing!
Warlock minions are a great class feature, and knowing how and when to use each minion is an important skill for any soul-draining Warlock. That said, I'd really like to see some improvements on this front in the near future, and thankfully Blizzard seems to agree. The improvements to their health were a good start, but there's still a long way to go for some of the less useful minions -- particularly the voidwalker, succubus, and doomguard. Giving the felhound more survivability would also go a long way towards making Improved Felhunter worth taking.