Imrahil has always been one of my favourite characters. He's everything you expect in a hero; brave, sensible, intelligent, loyal, wise etc. I think as Prince of Dor-en-Ernil he was an ideal ruler of one of Gondor's provences, down to earth and wise.
Like you said, it was Imrahil's cavalry that rescued the survivors from Osgiliath, and Imrahil who personally saved Faramir. After the death of Denethor he for a short while ruled Gondor and was rightly one of the Captains of the West.
I'm not really sure I would say he was underrated because he was given the above acolades. If anything, though, I'd perhaps say there was not enough written about him... for someone as powerful as he was, he didn't feature prominently in the book. There again, as he wasn't a member of the fellowship, he wasn't one of the main characters that the book was centered around.
To me, Imrahil and Faramir are very similar (as Imrahil was Faramir's uncle, this is perhaps not too surprising)
Yes, Imrahil wasn't an ordinary man, but just like Faramir and Aragorn, was another Dúnedain: meaning he was descended from the Númenóreans.
But beyond, in the great fief of Belfalas, dwelt Prince Imrahil in his castle of Dol Amroth by the sea, and he was of high blood, and his folk also, tall men and proud with sea grey eyes.
I made a wild assumption that high blood was Elvish which would mean that Imrahil is part Elvish and part Dúnedain.
"At length they came to the Prince Imrahil, and Legolas looked at him and bowed low; for he saw that here indeed was one who had elven-blood in his veins. 'Hail, lord!' he said. 'It is long since the eole of Nimrodel left the woodlands of Lorien, and yet still one may see that not all sailed from Aroth's haven west over water.'
'So it is said in the lre of my land,'said the Prince..."
The story goes that one of Imrahil's ancestors had a sinda wife.
I can't find any information as to whether PJ is leaving Imrahil in the storyline for the movie. I, for one, hopes he does.
I've always liked him. I think Tolkien introduced him at exactly the right time in the book - when all seemed (almost) lost, and the (human) main characters in dire straits - Aragorn off recruiting ghosts, Faramir about to be struck by a poisoned dart, Denethor gone bonkers, Thoden dead & Eowyn out of action....and he was calm, solid, good & true. I thought that making him the temporary ruler (although Gandalf called the shots) was one of the best political moves in the whole book.
The paragraph about his meeting with Gimli & Legolas has to rate with some of my favourite of all time.
No, seriously now, I do think he plays an important role in the book, but he's just not the kind of character I pay a lot of attention to in a book. In any book he would have gone without reading with me, I think. I just passed him by every time.
I hope the above is correct and answers your questions Naira.