A faithful scholar
I highly recommend Approaching Zion. I am a huge Nibley fan. Although I admit I have often been too lazy to read him as much as I would like. Lately I have been listening to audio of his lectures, which is a much easier way to absorb it, I find.
The thing about Nibley is that you know he has a solid testimony and is a faithful member. He does criticize church culture sometimes. But I have never heard him criticize the prophet or apostles directly. He gave a lecture about this topic titled "Criticizing the Brethren" - as in, don't do it. He sustained his church leaders, which is the mark of a faithful Mormon. Do we as members have room for improvement? Oh yes, we sure do, myself included. Nibley is great at pointing out ways in which we follow Babylon culture instead of living the gospel. Approaching Zion points this out very clearly.
He always realized that faith comes foremost. Despite a lifetime of scholarship, he said it that wasn't actually the most important thing: "We're just sort of dabbling around, playing around, being tested for our moral qualities, and above all the two things we can be good at, and no other two things can we do: we can forgive and we can repent" (Faith of an Observer, 1985 FARMS film).
Nibley was a great LDS scholar, as others have commented. The continued support of the BYU Neal Maxwell Institute (formerly FARMS) is evidence enough of that.
I happen to have known some of the students who fact-checked Approaching Zion when it was published. One or two commented that he sometimes took some liberties with his sources and/or didn't fully cite them. But their overall opinion seems to have been that he was a good scholar, and that he had blazed many paths that others would later pursue further.
For me, Nibley is a model scholar-saint. He is both, as we should all be, not just one or the other. Nibley shows us how it's done.