The abridged version: Lord of the Rings, but Welsh.
The less-abridged version: Really, Prydain‘s biggest flaw is that Lloyd Alexander had most of the ideas that countless lesser writers decided to ape from. Prydain was first published in the 60s, about ten years after Lord of the Rings was. The Fantasy genre barely existed back then, so most of these ideas (orphaned hero, headstrong princess, spooky, inscrutable bad guy with an artifact of unspeakably spooky power, etc, etc) were a lot more novel and impressive back then they are now, after a couple generations of lesser writers standing on the shoulders of giants. Most of what happens in Prydain will seem familiar and played-out, and it is, but only because the people who came after are the ones who played them out.
Prydain has a clear influence from Welsh mythology. Compared to the ubiquitous “Generic Quasi-High Mediaeval English Fantasy World”, it’s not something I’ve seen much in other Fantasy series. That means, even though most of the plot beats are pretty familiar, the world in which that plot is beating is fairly unique. Though, Welsh being, um, Welsh, the pronunciation of the names can be a little daunting – can you pronounce “Eilonwy” without looking it up? Though the edition I have contains a pronunciation guide in each book.
Also working in Prydain’s favour is Lloyd Alexander’s writing style. Again, though the things he is describing are all things we’ve seen before, at least he’s doing it in an interesting way. His dialogue and character interactions are fantastic. The aforementioned Eilonwy’s mannerisms and speech patterns lead to several hilariously turns of phrase and one of my favourite personalities in any book I have read.
Prydain is sort of a midpoint between The Hobbit and The Lord of the Rings, both in terms of reading level and thematic intensity in terms things like of violence and subject matter. It might be a little much for younger kids to handle, both in terms of comprehension and subject matter (major characters die, especially towards the end), but a Junior High student shouldn’t have too much trouble with it.