I have worked with UDC. Both stacking shelves and cataloguing. Its the peak of classification systems in my biased opinion.<p>It gives a lot of power in organizing a collection to local needs and specifications. The class numbers can be made ridiculous specific if needed, a good trait for academic collections that can usually be very subject insular. When you get a feel for the system you get an whole other insight when browse shelves. Need to find books about Italian law? Go look where 340-350 is shelved and grab the books with (45) on their back. Upon getting a request from a user, without looking anything up, you can interpret the subjects of the request and often walk right up the books that'll satisfy the user.<p>That said it takes effort and training to use, and applying it to an existing collection - instead of building one UDC from the start - is a mammoth task no one is willing to waste man hours on. Time is moving on and library classification schemes are a dying tech.
Now we have tagging and full text search, such limiting hierarchies are no longer that useful.<p>In libraries where the stacks are closed, what would even be the point?
The classification system works, sort of. But...<p>It comes a moment after going to the library a number of times when you realize that human knowledge is _not_ hierarchical, a tree - it is a graph. And then you understand why it is a monumental task to make the classification to match people's expectations.<p>You can't map a graph onto a tree.