if you are reading more than you are writing, this method seems pretty cool.

Actually, most writing functions are easier too ;-) Update operations on subtrees and leaf nodes can usually be implemented in a single SQL statement.

Insertion is really the only vaguely complex/expensive operation - and that's still just three SQL statements in a basic implementation.

The only downside is that inserts can be expensive compared to just storing the parent ID. With the implementation Celko outlined in the article inserting a node is O(N).

Still the average case isn't that bad. You can also make inserts less expensive on average by using increments larger than 1 and inserting in the "gaps" until a global renumbering becomes necessary - but since this makes the selects more complex, and most systems read/update a lot more than they insert, it's rarely worthwhile.