Umberto Eco's reading of Aquinas is sympathetic, far from the extremes of uncritical Thomist fanpersonship (which is how he characterizes vast swathes of the existing literature at the time of writing in 1956; even the neo-Thomists he chooses to cite receive significant pushback) or offhand kneejerk dismissal of all things medieval, or in fact everything written about aesthetics before 1954. He teases fascinating ideas out of superficially unprepossessing material: Aquinas's ideas about beauty are scattered throughout his work, not set forth as a unified section of his unifying works, and briefly quoted may seem simplistic to the point of inanity–"that which pleases when it is seen", indeed.
By situating them within their proper context and striving to illuminate them, Eco may or may not convince you that these ideas have any merit in themselves. In his conclusion, appended at a later date, he proposes a more modest and subtle aim of the foregoing work:
Whenever philosophy claims, "This is how things really are," it performs an act of mystification. Aquinas's image of an immutable reality was mystificatory, when confronted with facts that demanded something quite different. The image of reality found in modern aesthetics is also mystificatory, when confronted with other facts. […] If we cannot say that one system of thought is more or less true than another, if all systems represent the effort to rationalize the historical relations found at a particular moment in the development of Western society, then they are all of equal value. Medieval and modern aesthetics are of equal value, provided that they are taken as explanatory models to point us in new directions, as a machinery with which to face the problems that now confront us.
This may not seem like sufficient inducement to wade through 200 pages of extreme abstrusity; in which case, if you are nevertheless a fan of Eco's more popular works and mildly curious about this one, the conclusion alone might suffice in a pinch.