I really like Ikit's fluff, and so I like fielding him when I can. However, he tends to not be as useful as I'd like him to be.
It's true that for straight up casting power, a Grey Seer is the way you want to go. However, as previously mentioned, he's marginally more survivable thanks to his 3+/5++ saves. And, while only having two attacks, he'll still be able to dish out some hurt, since they're S6 attacks.
Assuming you don't roll Warp Lightning as one of his spells, he will effectively have 4 spells with the Warp Lightning from Storm Daemon. While it's nice that if Storm Daemon miscasts, it won't harm Ikit at all, it also means that Ikit will be down a spell and a magic halberd for the rest of the game. Another nice feature of Storm Daemon is the (small) chance of adding D3 power dice to the Skaven pool. Not something to rely on, but a nice bonus when it happens.
In my opinion, Storm Daemon is why Ikit costs a little more than he's worth. In the previous edition, you didn't need to use power dice to cast bound spells, so he essentially got a free crazy strong Warp Lightning.
I don't think anyone would argue that Ikit has to stay inside a unit (like all good Skaven leaders). This way he's shielded from missile fire, and forces your opponent to engage him in combat. This is where Ikit's Warpfire Thrower comes in handy. Using it as a Stand and Shoot reaction will most likely cripple the charging unit since he gets to reroll the artillery die. Even if it flops, even a single wound will force a panic test.
In the end, I'd have to say that for tournament play, Ikit should probably stay home, but for friendly games, he's got enough punch and variety to be a lot of fun.