It acts like a quest-based follower, but you don't ever drop it off after getting it. It provides some bonuses for all warlocks in your party, and nothing for any non-warlocks. I don't know about the spell skill bonus, but it does provide slow mana regeneration; I'm under the impression the spell point regen functions the same as the item ability that slowly regenerates a wearer's spell points. So your warlocks don't need anything that regens their mana over time, since the two effects don't stack. I've never tested it out much, so I may be mistaken about the stacking. Getting mana regen items for everyone by the time you can get the familiar isn't an issue, regardless.

The Light side doesn't get anything like the familiar. Of course, the Light side 2nd Druid promotion quest isn't nearly as much of a PITA, and can actually be done quite simply right after you go Light side. If you hang on to random quest items that you find, then you might even already have the item you need to complete the quest before you get the quest itself, and the Archdruid quest locations are infinitely easier than the Warlock quest locations.

It isn't even a contest that the Warlock promotion quest is one of the hardest promotion quests, if not the hardest. The 2nd Knight promotion quest might be comparable under certain extreme circumstances, but the average player isn't likely to run into those circumstances, and a player who is already familiar with it won't have any trouble with the Champion promotion quest at all.

I'd argue that the Ninja promotion quest is up there too, not for any particularly difficult battles you might have to fight (as with the Warlock and possibly Champion quests), but for its moon logic.

Of course, a half-decent guide on GameFAQs could just tell you how to complete it, but where is the fun in that?