It was highly fitting that Sophocles would end his long, prolific life with a third Theban play focusing on the tragic events surrounding Oedipus. Chronologically, Oedipus at Colonus followed Oedipus the King and precedes Antigone, providing an important link. It is substantially different from those works, and though significantly less great, the fact that Sophocles could make such a quality work in his ninetieth year after so many excellent ones makes him almost unique in art history. Most writers would do well to turn out a single work on its level. This swan song remains an immortal work of world literature and is essential for anyone familiar with the more famous Theban plays.
As in Oedipus the King, the character of Oedipus may be the aspect that has always spoken most strongly. He is one of literature's most thoroughly sympathetic personages, and the truly pathetic depiction of him here as a broken old man near death - blind, seemingly at least partly senile, and dependent on his daughters for even the simplest tasks - may be even more moving than his downfall in Oedipus the King, powerful as that was. Here he is reduced to the most abject misery possible to humanity - a state so pitiful that even reading of it is nearly unbearable. Though he had clear faults even in his prime and caused his own decline, it is virtually impossible not to sympathize with him; he is truly more sinned against than sinning. He has flaws even here; his impulsiveness has increased, his temper has shortened, and he lashes out at people - including his own sons - with little provocation. Yet he remains sympathetic; such things if anything make him even more human; we feel for him because we see his profound humanity. However ostensibly different from us, he has the indisputable human core necessary for a truly moving character. The play is valuable for showing the nadir to which people can sink even after the worst has seemingly happened, bringing out life's inherent tragedy with incredible force and emotion. His state is indeed so low that he is forgiven by Zeus and allowed to die not only with dignity but with some satisfaction at a return of his importance after decades of pained exile. On top of everything else, the play is a thoroughly moving depiction of true compassion and noble forgiveness. Despite many dark moments, it is uncharacteristically optimistic for ancient Greek drama - indeed no tragedy at all, though ostensibly styled one. It suggests that there is always a possibility of at least partial redemption and underscores the profound significance of empathy and mercy. Sophocles' nearing death may have brought on such thoughts, but their universality makes them timeless; the play continues to speak at least as powerfully as the tragedies to those willing to listen, and its greater palatability makes it potentially more relatable.
Unlike the prior two Theban plays and Greek drama generally, Colonus has very little action. It is essentially an emotional drama that works via dialogue, but there is also substantial philosophical dramatization. The grand themes and monumental speculation of Oedipus the King and Antigone are mostly gone, but it does handle important issues like the responsibility of parents toward children and vice versa, questions of political succession, society's treatment of outsiders, the significance of ritual, etc. Those who value the first two plays for taking on weighty issues more grandly and overtly may be somewhat disappointed, but this still has a good amount of weighty themes, and its elegiac aura is in its way even more emotional.
All told, though Colonus is not an indisputable masterwork like Sophoces' more famous plays, it is well worth reading for anyone who enjoys the latter, though Oedipus the King and Antigone should certainly be read first. The real question is what translation to get. Robert Fagles' is undoubtedly the best for current readers. It is not that prior ones are inaccurate, but inevitable language changes have made them ever less readable; some may think them more stately, but they lack Fagles' flow and readability. Dedicated Greekless readers will of course want several, but neophytes should start with Fagles, the only version most will ever need.
Translation aside, the question of what edition to get is even more pertinent than with the first two plays. Unlike them, it does not stand well on its own, making an edition with the full trilogy essential. Standalones are hard to justify unless one wants a deluxe edition with Greek text, extensive criticism, or some other bonus. Everyone but the few seeking such things should get the trilogy.