I'm pretty sure that although I started The Prydain Chronicles before, I had never actually read my way to the end of the 5-book series till now - it's a well known series, probably one which acted as a gateway into fantasy books for many readers and which also led to one of the less well known Disney movies, taking its name from the second book. As part of a recent TBR challenge, I took on reading the entire series as my candidate for the book that had been on my TBR list for the longest and managed to finish it off yesterday.
Overall, I'd say that (compared to some fantasy books which are its contemporaries anyway), The Prydain Chronicles ages fairly well. For those who've never read it: the protagonist of all the books is Taran, who we first meet as a stroppy teenager who dreams of doing heroic deeds to take himself away from the reality of his life looking after Hen Wen, a white pig which can tell the future. Alongside his wish for valour, Taran doesn't know where he comes from as his mentor and substitute father Dallben refuses to tell him the truth about his origins.
Taran is subsequently drawn into a series of adventures since his pseudo-Welsh homeland is under the rule of Arawn and his deathless hordes, unkillable soldiers created by the cauldron of book 2. The first quest starts with the disappearance of Hen Wen, for whom Taran goes looking and which whets his taste for adventure; book 2 has Taran and his friends attempting to capture the cauldron, while book 3 is about his friend Eilonwy being sent off to become a proper princess and the problems Taran is dragged into alongside this. For me, book 4 is possibly the best of the bunch as Taran starts to grow up in more ways than one, realising that his desire for glory is actually quite hollow, while the final book ties everything together and sees the defeat of Arawn once and for all.
There's an ongoing theme, especially through the later books, of the value of heroism and the importance of doing what's right. Those books in particular also value people doing their best at whatever they turn their hand to, especially more practical tasks like smithing and pottery. As with many fantasy books, Taran lives in a world full of noblemen but also one where a kind deed years before can be rewarded at an unexpected moment.
If there's one thing that doesn't age particularly well, it's the classic fantasy trope of the lone girl who is Not Like Other Girls: Eilonwy is a classic example of this and the degree of petulance at times can get a little wearing, as can her deafness to the class differences between her and Taran. She's a princess and can choose when she wants to be treated like one, while Taran is literally the child of nameless parents and doesn't have the same luxury in that society.