Have I got this Right

[Grandorf]

Ok :
1. Use imp for instances and travling. (1st rule against hunter/warlock, Ignore pet kill pet's master. So VW doesn't work well in pvp)

2. Use Felhunter in PVP.

3. Use scubby in some instances


Please elaborate on these.

*** Information added 0 Minutes and 56 Seconds later... ***

Forgot:
4. Use VW for most lvling situations.

[Soulbourne]

I use imp for pvp and groups. I sometimes use for soloing when I don't mind healing occasionally but it's not a tactic I suggest for everyone. Use VW for soloing mainly. Succy....I sometimes use just because, but never really see much use. Felhunter, pure pvp, low damage and no threat as tank. Just be sure to control spell lock instead of auto cast.

Felguard, fairly general.

Infernal: I go FG, Feldom FG, Infernal, and if they blast through all that probably hear the song "what was I thinking"

Doomguard...for extended battles curse is good but I hear he is horrible.

[Pure Balance]

Those are very basic and not in depth things.

I use succy in most 5 mans because even if you have adequate cc, she makes a very good interrupt on normal mobs throughout the instance. In raids it all depends on your spec. Obviously if you're 0/21/40 you're going to sac a succy of imp dependent on if you're fire or shadow. If you're 0/40/21 you're going to want to use an imp typically for more spell damage through DK(espcially if you're improved imp) and threat reduction through Master Demonologist(much less the stamina for you and your party). If you're affliction I think they always use imp for everything pve for dark pact.

For pvp it all depends on your play style. I use succy most of the time because it helps you beat any class whereas a felhunter will not do much against a warrior or enh shammy and only really helps against rogue and druid stealth. Succy can also stop mage/priest/pally/ele shammy casting midcast if you're good and get the seduce off before they can cast(seduce is usually just barely faster than most of their spells especially if you have a Curse of Tongues up). Granted some people will be more apt to try to kill your succy than your felhunter, but in the time they do that you could be hurting them more and still be at full hp by the time they kill it.

[dogwatch]

Your pets are very situational.

Best pets for PVP, Arena, WsG, BattleGrouds etc. is Felguard or Felhunter.

General grinding and questing around the lands I would go with Felguard or Voidwalker. Both can hold good agro and keep monsters off you when your rolling solo.

5 Man instances: Some cases Succy for crowd control or Imp for Blood Pact or Felguard,Fehunter may also work good in certain spots.

10-25 man raids I use Imp, Felguard, or Felhunter all depends on on what boss your on some pets are better than others.

[Eterud]

Pre The Burning Crusade the imp was your answer to all,
now I take for pvping an succubus or felpuppy.
Felguard is nice for pvping aswell, vs clueless people.
However they sacrifice so much that they are pretty easy killable themselves.(heavy trouble from spellpushbacks.)

However vs warriors in just duels and such I use the imp still,

nice dps, nice extra health, while you can burst the warrior down.(for pvping I'm the 5/7/xx firespec, for raids 0/21/40 sb only spec)

in instances I normally sacrifice my succubus,
the damage boost>imp on any way.

[dogwatch]

Ok I am a demon spec here and I just don't get the sac the succubus thing at all. I know your getting a +10 shadow damage bonus buff when you do it.

However when I summon my pet my spell damage jumps up considerably plus soul link (demon takes away damage from me) plus my resistances jump from 0 to 35 each or even higher depending on what pet I have out. I dont see how that minor 10% buff is helping me in any way as a demon spec my stats are far worse when i sac my pet.

[Malevevely]

Being full demo you have more talents then the people who are saccing the succy. We run with DS as the highest tiered demo talent. This means no bonus spelldmg etc from having a pet out. SL is also on a higher tier then the DS talent hence saccing is the best way to get the most damage out of your pet.

[Balroq]

Exactly correct, you wont be saccing your succubus if your demo spec. Additionally the succubus sac is 15% bonus shadow damage not 10. So if you decide to go 0/21/40 thats when your gonna see a damage increase (Even though technically your NOT going to see it in your stats)

Now to the OP:

Like everyone else seems to be saying its not only situational but it depends on your spec. If your aff IMP is always a good bet PVE. PVP your looking at succy for CC or felhunter against heals/casters. And if your 21/40 in raids your gonna sac the succy for the bonus.