Should the title be poor, bear with me for the duration. I've been reading him recently mainly for two reasons; 1) I'm interested in the best student of G.H von Wright (and the connection to Wittgenstein through him), and 2) I happen to be finnish, I studied philosophy for a while, and sadly, skipped my only chance of meeting the already elderly Hintikka, didn't know much about his value then.
Anyhow, I must admit that plenty of his technical papers are simply too difficult for me. However, I sort of see plenty of problems in his "vernacular" (for the lack of better word) work as well - that is also quite difficult - but I don't feel completely out of depth there. Some things that annoy or puzzle me are;
He is very dismissive of many highly competent philosophers (I do forgive him for his take on Rorty, though! /s). One example is dismissing Hartry Field in as few as two sentences.
He seems grossly overconfident, almost arrogant at times. Especially with his insistance on the significance of IF-logic. He insists on its priority without addressing the negatives (like losing many metalogical virtues while only introducing few).
Having been included in "library of living philosophers" suggests great significance, but I sort of fail to see what it is, exactly. Introducing modal logic semantics independent of Kripke is a big thing, of course, as is game-theoretical semantics. But those are old, what else is there? Do people still engage with Hintikka? In my understanding, Väänänen has managed to re-orient IF-logic rather constructively, but I cannot think of too much else.