The Princess and the Goblin is one of the first books in the modern fantasy genre.
This book had tremendous and very visible influence on all the (now much more famous) authors that came after it. It is of course very dated. It does not match the standards that fantasy have created since; in neither scope, story, characterisation or complexity.
On the other hand, it is a rather enjoyable little fairytale, and it does have its positive sides. Mostly, though, this book is not really one you read for fun.
Except maybe if you're eight years old and have never read a fantasy book. If that is not the case, this book is more an object for study than a source of entertainment.
It's a satisfying read because it gives an impression of what fantasy was like in the 1800s, but that unfortunately also means that it gives an impression of what fantasy was like before it got really good.