I don't really like the Djinn, of course, his spells are powerful, but without them, he's quite weak. He rarely proves useful against a hero, and when it comes to defeating enemy armies... well, he struggles, especially against, for example, Nature creatures or Chaos.
He's actually perfect against Chaos. And Nature won't put up much resistance either. By the way, don't forget about illusions... In the end, he can use all his abilities for that.
And one more thing... there's no worthy replacement for the Djinn. Not only is he the best of the 3rd-tier creatures, but there's also no decent alternative for him in the castle! Nagas vs. Djinns? Does anyone doubt it?...