The way I look at the Pentagram compared to the other spawnpoints is like this:
Gate to Voltari:
14 mana to gain 1-3 mana per turn (most likely 2-3 but a mage casting no spells in a turn happens occasionally), with the added benefit of making your opponent not want to cast spells.
With the mental effect on your opponent and several good creatures that it's capable of summoning, it is a good spawn point.
Temple of Asyra:
10 mana for 1 mana per turn, can spend 5 more mana twice to gain +1 mana per turn.
I consider this to be the most expensive spawnpoint. Yes, the initial cost is 10 compared to 14/15, but for the other spawnpoints getting 2 mana per turn with them is not a stretch (Lair is base 2, Gate gets 2 if opponent casts 1 spell, Pentagram gets 2 if you hit the opponent's mage once with any creature) and to get two mana per turn it requires 15 mana spread out over two actions. Additionally, the Gate and Pentagram can potentially get 3 mana per turn reliably against the right build (swarm for example) and for the Temple to get that it requires a third action and another 5 mana.
Now, on the other hand the Clerics have some minor utility for other things (though they have the worst attack of any creature and the healing is fairly negligible [an entire action for 1 damage healed on average does not seem worthwhile]) and the Temple itself has great synergy with the
Temple of Light. Combined with the really powerful Holy creatures and the Priestess' power in the late game it can definitely be worth it despite the high cost in actions/mana to get it running.
Lair:
15 mana for 2 mana per turn.
Fairly expensive (second only to the Temple) but definitely worthwhile for the Beastmaster due to the wide variety of creatures it can summon (especially for swarm builds).
Pentagram14 mana for 1 mana per turn, up to 2 more mana per turn with successful attacks.
As a spawnpoint, I don't consider this one to be the "worst". For a build that would use the Pentagram multiple creatures are likely, so even against creature-less builds it is fairly unlikely that all of them would be stunned/daze/miss (for example, a Forcemaster using the new ring that boosts defense die rolls by one with Cobra Reflexes, Bracers of Deflection, Force Orb, and Force Sword would give a 5% chance to negate 5 attacks in this way), so getting 2 mana per turn on most turns is not unreasonable. Also, against builds that field multiple creatures getting three mana per turn out of it is almost guaranteed (as Firestorm, Ring of Fire, or Adramelech can each get 2+ attacks in for one action). So as something that summons creatures I don't think it's that weak. Ignoring what the spawnpoints can summon I'd place it at about the same level as the Lair, with the Temple and Gate being the two best in most circumstances.
The problem with Pentagram is that it can only summon
Living Dark creatures, which throws out the Skeletal Sentry. For Dark creatures under 10 mana worth summoning this leaves the bat (the imp isn't worth it in my opinion). After that there are the two level 3 demons which are fairly good (though they aren't quite as good as one of the level 4+ dark creatures and cost almost as much). Then of course there is the vampire, the werewolf, and the two named demons.
So all of the creatures worth fielding are fairly expensive, except for the bats, so in order to get full use out of the Pentagram is requires you to go into the late game. And here is the big problem...the Warlock is strongest in the early to mid-game, with the other mages getting comparatively more powerful as the game takes more time. This means that instead of spending 14 mana for a long-term benefit, it is better in many cases to spend that mana now for an immediate beatdown (like a turn 2 or 3 werewolf in your opponent's face).
Of course, this problem will probably be solved when more expansions come out featuring more low-level dark living creatures (and hopefully more demons) and perhaps some more cards that give benefits to demons, which it honestly feels like he should have (more things like Gate to Hell would be cool).
Summary: While I don't think that the Pentagram is one of the best spawnpoints, I would put it on par with the Lair in terms of mana generation. The problem I have with it is that the summonable targets either are too weak or are extremely-powerful beasts that are better to summon immediately instead of waiting for the Pentagram (or any spawnpoint) to generate the mana required to bring it to the arena.
