Any book where children have to do with a door to a magical world has to contend with comparisons to Narnia, especially if the children are two boys and two girls, and the door is via a magic wardrobe. That said, there's enough that's different here for the book to stand on its own. I know that the book worked well for me when I was young, though I don't recall if that was pre or post-Narnia. I also didn't recall until this read how well this (with its enigmatic stranger) set me up for Thomas Covenant (which this book predates).
Other than the mechanism (which is different in detail), this story isn't really that similar to Narnia. For one thing, almost all the action takes place in the real world. For another, the wardrobe appears to have quite a different function at first.
While the story worked for me as a child, and I strongly remember the impression it left (if not much more), as an adult, I was less taken with it. There's quite a lot of deliberate symbolism, but it doesn't really add up to much, and the story ends without many of the mysteries explained. It's more like the first half of a story than the whole thing. It does wrap up a bit at the end, and apparently it was enough for a child. This time through, though, it left me unsatisfied and disappointed at what I remember as a very good book.
All in all, a fun light read, but perhaps best for young children who will accept vague symbols as significant mysteries.