Overall, I think I prefer single volumes over an omnibus--that holds true for most books, not just the Narnia set.  For one thing, a single volume is less ponderous for holding in your hand; I don't read lying down very often any more, but a one-book edition is rather heavy going.  For all that, I did use (someone else's one-volume copy, softcover) to read to a class last year, and didn't mind, since a desk was available.

Probably the best reason for individual copies is the cover art.  Several years ago I put together a collage of all the different covers I could find for each title--there are quite a few, and some of them are really nice.

One more question: if you're counting the number of books you have, does an omnibus count as one book, or the number of stories combined in it?

@aileth 

If you have all seven of the stories in one book I would consider it to be one physical book with the pages, binding, and cover. But if you are looking at just the stories it is a collection of seven books. There are two ways of perceiving the concept of a book. It depends on your point of view. 🙂

Yeah, I guess that's an accurate way of differentiating.  As I'm packing up our library, I'm trying to keep a book count (how many in each box) as well as an author count (how many works per author).  Therefore, the totals may be quite different, since multiple editions of the same book are one work, but several books, and vice versa.

Also, compilations are often abridged or otherwise messed up, but I've never heard complaints of the sort for the Narnia ones, so that's good.  As you mentioned, if you're fussy about the order, it must be maddening, but then I do keep my (wrongly numbered) set in chronological order, while recommending publication order to new readers.  I'm sure there'd be some kind of a market for a "properly" numbered set or compendium, but maybe not strong enough to be financially feasible. 

I remember seeing a one volume hardcover edition of all seven Narnia books with color illustrations by Pauline Baynes. It was in a Christian bookstore many years ago and cost around $50. I didn’t buy it because I already had the separate books in hardcover and had spent a lot of money on them. But I think that edition of Narnia was one of the prettiest books I have ever seen. It was even better than the Barnes and Noble edition from about twenty years ago. I think both of those beautiful books are out of print now and probably very expensive.  They did have the disadvantage of having the seven stories in chronological order, but I think that they were worth the lower price when they were new and in print. I am not sure, but there may have been a one volume edition of the Narnia stories in publication order before the 1990’s. If it does exist it is very rare.