Familiars will do good damage against anyone with low armor for the first one or two hits, but after that the damage falls off sharply, and due to their low durability and high bounty, they don't provide much DPS since they can't stick around. The only reason to even take familiars is for the stun, and as far as the stun goes, personally there a lot better options.
It's seeming like there are a lot better options in general than Visage for almost anything except trilane versus trilane, and even then I think having a Venomancer would be better.
---------- Post added 2012-09-01 at 01:14 AM ----------
I've never had a familiar die to an enemy player and I haven't missed any stuns with them on anyone I was aiming for. I don't have issues using them, I have issues finding them to be worth picking Visage for, which is apparently the primary reason to take him if you aren't trying to dominate an enemy trilane.
Visage is amazing if he can just chill in the middle of team fights and just constantly shit Soul Assumption everywhere, but that both demands a lot of mana, 140 every 4 sec, and also assumes the enemy team is just going to let him sit around. Gravekeeper's Cloak makes him really hard to immediately blow up with something like Laguna Blade, but after four hits from any hero (including dots from heroes like QoP, Veno, and Viper) cloak is completely removed and you're left with a slow INT hero with the same godawful base armor as heroes like Doomy and Tiny, but without their high STR, and without any escape abilities.
Basically, it seems like Visage needs to roam a lot in order for maximum effectiveness - especially if the enemy team isn't stupid and doesn't try to fight a Visage trilane with a trilane of their own - but there are a lot of better choices for that kind of playstyle. Soul Assumption is great in team fights if you can stay alive and have the mana to spam it, but if I wanted a hero that ganks and tosses out a constant stream of nukes while also being hard to burst down... wouldn't QoP be a better choice?
I would rather have Lycan's wolves than Visage's familiars every single time. They're better in all the ways that really matter - much higher DPS, more health, considerably less bounty if you happen to get them killed, and they synergize with Lycan and especially Lycan's typical item builds (especially Vlad's) a lot more than familiars synergize with Visage. On top of that, they're also fantastic scouts due to permanent invisibility and pretty damned good 1200/800 sight range, and while they aren't flying units, they move much faster than Visage's familiars, up to the point of moving at maximum speed and gaining immunity to slowing effects during Lycan's ulti.