He's pretty fun to play with. The only thing you have to keep in mind is that he is purely offensive. If you are a person who relies on buffs from the Slann, he might not be your favorite model. Several people have been running "Kroak Bomb" lists (myself included) and admit that they are fun when they work. He also makes your TG Unbreakable, so that's a huge bonus. For what he costs he is good. If we get a FAQ that lets his spell hit enemy units in Close Combat he'll be competitive I think. The boosted version with the 24" range is devastating against T3 troops. Just remember to bring a Scar-Vet BSB to pick up the slack.
Besides being the king of badassery in the fluff, Id say slann is more competitive but kroak is still viable.
So much depends on FAQ clarifying the one direct damage spell. Is it a bubble? Can it hit all units within range, even if in combat? The word "all" suggests bubble and over-riding the BRB restrictions on DD spells (can only target units in forward arc and not in combat). That one spell in a grinding match battle becomes huge, especially if against undead or daemons armies. The abilitty to avoid the miscast effect of killing off your own Temple Guard and ability of the TG to survive to the last man are both useful, particularly if the Slann or Kroak declines the challenge (with no other candidates) and you lose the general LD or BSB benefit. A common reason for loss with a larger TG-focused unit is having the TG unit so depleted by miscasts that it cannot survive sustained combat and the Slann ends up by itself, breaking and then being run down. Add to that the -1 to hit the TG unit with Kroak, and the TG unit becomes a rock. The unit will win more combats and win the combats it wins by a lot more. Armies with BS shooting (empire hellblaster volley guns, high elf archers+ repear bolt throwers, dark elf crossbowmen+repear bolt throwers) are coming back and use that shooting to deplete TG units. Being an unbreakable unit has a lot of value. Kroak is most useful as a point denial general. Whereas a bad miscast often is the primary reason a slann focused army loses, that will rarely happen in this army. With t5, 6 wounds, minized miscast damage risk, and a 3+ ward save, he will rarely die unless the TG unit he is in is completely destroyed or someone gets a lucky killing blow or multiwound hit (especially with flaming). With the reduced miscast damage consequences and the -1 to hit in shooting and combat for his unit, the TG unit is much harder to kill off. He probably needs to be run with skink priests as scroll caddies (cube and/or dispell scorll) for defense, their own spell casting abilities, and a direct damage vehicles for Kroak's spell. Given that special characters are allowed running Tetto'eko makes a lot of sense as a strong caster with loremaster heavens and abiility to cause all 1's rolled in attempts to cast to be re-rolled. Taking away the rolling of 1's really smoothes out the odds and reduces the chances of a loss in concentration but at a heightened risk of miscasts. For each dice you throw at a spell, the average casting value increases from 3.5 to 3.917 plus the really low casting results tend to go up while the really high casting results tend to not improve at all or much.