The siege of Rhodes was hardly Demetrius Poliorcetes' only siege, and though Demetrius eventually lifted the siege Plutarch says that the Rhodians were extremely impressed by him and even asked to keep some of his siege engines as a reminder of the sort of man they had fought against. Among the cities that Demetrius laid siege to were Messene, Thebes (three times, they revolted several times), Athens, and famously Salamis in Cyprus. Plutarch says that Demetrius was particularly famed for his engineering works, including his enormous sixteen-banked galleys and his massive ἑλεπόλεις. You notice I use the plural there--the Helepolis at Rhodes was not his only one, just his biggest (and probably this engine alone is what gave him the name Poliorcetes, it was truly enormous). Demetrius built his first such engine during his siege of Salamis, and the one built for the siege of Rhodes was even bigger and more impressive. The Rhodian Helepolis was pretty truly magnificent, we have descriptions of it in Diodorus and Vitruvius, and pretty definitely worthy of granting the man a title for the brilliance of his engineering feats. In other ways Demetrius at Rhodes earned the title Poliorcetes. Though he did not succeed in taking the city and probably suffered more hardship than he inflicted, he did negotiate a more or less favorable treaty, breaking down the alliance between the Rhodians and the Ptolemies and forcing the Rhodians into alliance with him (except in the case that he warred against the Ptolemies). Of course that's not really what Demetrius began the siege for and Plutarch says that he really just wanted an excuse to get out of there, but it's not as though he was totally defeated and driven out. It was more inconclusive than anything else. And the siege of Rhodes (and the triple siege of Thebes, to say nothing of the siege of Salamis) was pretty epic, in the Homeric sense. The acknowledgement by the Rhodians of the incredible nature of the siege has already been mentioned, but Plutarch tells all sorts of stories about what happened there. Most famous is the Helepolis, but Plutarch also mentions Demetrius as wearing armor that had been tested by firing a catapult bolt at it, and Demetrius' declaration to the Rhodian envoys that he would sooner destroy his father's image than Protogenes' painting, which he had captured on the outskirts of the city. So despite Demetrius' rather mediocre performance at Rhodes the siege was beyond memorable, it was almost mythical in the memories even of Demetrius' contemporaries. This, together with his various other sieges and his fantastic siege engines, I think is enough of a justification of his title