In my experience as DM and occasional player, Black Tentacles is nigh-deadly against any non-teleporting corporeal opponent of less than the caster's level, and against primary spellcasters of most levels if they can't Teleport (or Dispel Magic stilled or spell-like). This does assume that the party gets in a round or two of ranged attacks and spells after the oppenent is grappled.
My party wizard has been having entirely too much fun with Black Tentacles, so last game (an assault on a Drow stronghold) I gave the spell to several of the Drow.
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
They only encountered one of them, but he was the #2 Big Bad, with a Caster Level 1-3 more than the party's levels. And he showed up while part of the party was still in a Webbed area from a previous fight (or an earlier phase of the same fight, really; the party went over 30 rounds without dropping out of combat, much to the annoyance of the barbarian, who raged early on and ended up fatigued for most of the fight).
Anyway, I'm here to warn you: Web + Black Tentacles = Very Bad (and I don't even want to *think* about Web + Tentacles + Silence). Even if you save vs the Web, which the party did. To quote from Web:
Once loose (either by making the initial Reflex save or a later Strength check or Escape Artist check), a creature remains entangled, but may move through the web very slowly. Each round devoted to moving allows the creature to make a new Strength check or Escape Artist check. The creature moves 5 feet for each full 5 points by which the check result exceeds 10.
So, if you're Tentacle-Grappled and Webbed, the sequence is:
On your turn:
1: Roll grapple check(s) to escape from grapple. Usually fail.
2: If you succeed, you've used at least one attack, so you *can't* try to move through the Web (it requires a full round to make the Strength check), and you can't do anything else except a move-equivalent action.
On Big Bad's turn:
1: Tentacles grapple you, usually win. If you were already grappled, take 7-10 damage. If not, you're grappled now.
2: Big Bad hits you with damaging spell.
On your turn: Lather, rinse, repeat.
If you can't Teleport (without the use of scrolls), your only hope is to beat the Tentacles' grapple check twice in a row (once on your turn to escape, once on caster's turn to stay free), and then hope you can make the Web strength check well enough to move out of the area of effect... and you'll probably need to do this 2-3 times, since your movement (probably only 5-15 feet anyway, due to the Web) is cut in half by the Tentacles (I'd give you a minimum of 5' movement, assuming you made the STR check in the first place).
They eventually dispelled the Web, but the Tentacles stayed for their full duration (13 rounds). Ultimately, they trapped the Big Bad in their *own* Black Tentacles spell, and he couldn't escape (most of his spells were used up, he only could have used no-somatic-component ones anyway, and while I was going to have him use the Scroll of Dimension Door he already had in-hand, a sharp-eyed player pointed out that the Grapple rules forbid use of spell-completion items while grappled.
Other issues I have with Black Tentacles:
No save + no spell resistance + area of effect;
Immune to all damage, apparently including Disintegrate, which just seems wrong. :\
A field full of 10'-long black tentacles, each square of which is roughly equivalent to a Large creature, but it gives no concealment or cover to those within or on the other side fo it.
On the other hand, we did decide that, since it grapples and deals "bludgeoning" damage, DR (something besides "magic") applies against it, which means that some creatures (and Stoneskinned characters) won't take the damage. They're still stuck, though, and likely to be spelled and arrowed to death in short order.