It's competent, but for all the hullabaloo in just trying to see the darned thing, not worth the excitement. Fontaine plays the same coming-of-age girl-to-woman shtick better in Rebecca and Jane Eyre (and to a degree, The Bigamist). I find the Gigi/Lolita story archetype just plain creepy.
The movie itself is so romantic (in all meanings of the word) that you really need to be in the mood for it. Joan Fontaine and Alexis Smith have some powerful scenes together...this is some of the best work either of them did. Well worth seeing, especially since it has been unavailable for almost 50 years.