I haven't played with it much because I usually play dwarfs, but after playing with it on my Slann for a Storm of Magic game, I decided to self-restrict myself from taking Becalming Cognition in friendly games. Unless I expect my opponent to take Teclis or a Greedy Fist/Death lore using Ogre, I don't plan to use Becalming Cognition. Unless I become convinced that's it's not unfair in our favor.
I do not think it is a bad thing. I also do not mind playing against Teclis. I think if you have the ability to use it, take it. The same thing can be said for how powerful cannons are. Will my empire opponent self restrict his cannons? I play Ogres as well and I almost always take the DeathFist build on my slaughtermaster. I am a casual player who plays for fun only. If anyone that I play complains about it, I will consider changing the tactic, but as of yet, no one has complained. My oppoenents usually try to stay out of the range for becalming (or the deathfist). While I try to get across the field, he will try to kill my troops with his catapults/cannons and magic that will hit me as I come. Ultimately, the Becalming Cogitation ability is just something that the Lizardmen benefit from until the cost is adjusted or the ability is changed when the army book is updated (sometime next year**crosses fingers).
I wrote a big spiel about self-limiting being appropriate so that everyone can still play in a way that they enjoy (in a friendly game). But I retract it all. It's interesting to try to get through a bastard build like a deathfist... once, maybe twice. The rub comes when you keep having to face the same difficult tactic, over and over again. Even if you find a way to consistently get on top of it, that is boring as hell. I think for me, the only self-limitation is to not stick with a good thing; move on, try something new. ... that said, I would never take cogitation in a friendly. Not that it's such a hard thing to get around. Merely because I know my opponent would instinctively just go "Ugh, #$%#ing slann. This is going to be a crap game." And even though it might not be true, it's really hard for someone to get out of that mindset, and they end up not enjoying the game.
All Becalming Cogitation does is punish people for relying on a single high level wizard to win the game for them. It's pretty weak if you have enough spells to put all your power pool to good use without casting with the Becalmed wizard.
We put our eggs in one amphibious basket; it better be a hell of a basket. With lore master, ethereal, rumination, cogitation, and soul of stone we have a bunch of great options. Depending on the matchup, cogitation isn't any worse than ethereal, or as universally useful as rumination. It does solidly put magic in our pocket though, if they can't deal with the Slann, except in certain matchups. Cogitation is just another of the great tools a Slann has to work with. It's not dirty unless they are inflexible.
I definitely wouldn't say that's "all it does". It's most effective at negating that style of play, but it will still potentially effect any use of magic. There is no "good use" when you need to waste points on an extra wizard, or factor in every time that you could potentially have a completely useless die. In a friendly, forcing someone to take multiple wizards to counter cogitation is an example of what I was referring to: having to repeat the same tedious tactic. When it comes to what tactics you choose for Proper Tournaments, anything goes, I guess; they have comp rules for a reason. However, some competitions have a point allocation for how fun/fair/enjoyable of an opponent you were to play against. Maybe not a big deal, but perhaps worth a thought. There's a fair amount of hate for slanns out there.
Dirty pool? Compared to what: Dwarves fielding umpteen runed up war machines that pretty much auto hit each round? Demons with an army wide ward save? WoC with re-rolling shenannigans to get a 3+ ward? Dark Elves with hatred + Mindrazor? I could go on, but I think you get my point.
Becalming Cogitation is simple to get around if the opponent knows how it works. Just make sure you're 24" away from the Slann; it's not that hard. My last game was against Ogres over the weekend. I squeaked out a win, but it was due to almost ridiculous dice rolling in the last couple of turns. I was running Becalming and the guy's wizard rolled 1 six the entire game, and his spell went off even without it. I have no pity for people who can't deal with any of our Slann powers; they are just not skilled players.
When I first have to tell them they are being becalmed I try and helpfully add that they can no longer miscast and are safe.
Honestly Becalming Cogitation should be the least of your opponents worries when it comes to the Slann. Cupped Hands Focused Rumination Higher State of Consciousness Those are the ones i here alot of people whine about but honestly the Slann is our bread and butter without him Lizzies wouldnt be so good. SO it makes sense to take him decked out. Besides i find taking a decked out Slann isnt nearly as bad as facing a plethora of other things other armies can dish out i saw someone give a short list a few posts up let me add to that a bit Double Hellpit warp lighting spam Skaven list (usually with incredible amounts of slaves) Double Hydra Dark Elf lists Warriors of Chaos Hell cannon spam High elves Arch-mage Book of Hoeth (ewwww) I mean the list goes on so atleast give your chubby frog some goodies so that way we dont get stomped by alot of other armies dirty business......
As a regualr opponent of Slanns, becalming Cogitation does not bother me at all. Cupped Hands is much more offensive. Hinge PS Lizards are plenty good with out a Slann.