At Cardiff University, we allocate Library of Congress subject headings (LCSH) to our catalogue records. This was not always the case: before we acquired our current web-based LMS in 1999 we did not do this, but with our LMS came lots of records from other libraries which did, and we also took part in a collaborative cataloguing project at around the same time which asked for them. Since then we have gone with the flow and tried to use them, with varying degrees of consistency.
I had used LCSH before, but that was many moons ago, when basic records of books were added to a computerised system for circulation purposes only and the catalogue was a separate beast entirely. The structure of LCSH is designed as far as I can see mainly for either a card catalogue or a printed reference book. The headings do not really make much sense when not seen in hierarchical form, and it's unlikely that many users actually search them in the way originally intended (browsing them in some sort of linear progression).
In the period between the card catalogue and the web-based OPAC, we did have a basic keyword search function, and over time the practice was to add simple (but completely uncontrolled) keywords to the records. As the system would not search words which occurred within the title of a book, very often these words were the keywords (now redundant, though many are still there). I no longer add them except in the rare cases where I really feel that only a British English word (or even a Welsh word) will be useful to our own users and the word in question does not appear in the title of the book. Sometimes I wonder why we go to so much trouble to find the right LCSH which, when found, turns out to be a term none of us would ever use. On the other hand, there are dangers in the freedom of the uncontrolled keyword too. Our older collections contain works couched in language which would not be used today, and in the past these books were sometimes catalogued by people who were not aware of various nuances and shibboleths and general opportunities for causing offence through choice of words. (I think I have now found and removed all the "610.20 Church of Rome" headings which I came across, likewise "Papists", and various other keywords suggested by the titles of the works of forthright early Protestant clergy).
As we move to the next stage, the new generation "resource discovery platforms" which are replacing the more structured catalogue, I wonder whether LCSH still fulfils its purpose, or whether we should once again be looking at something less rigid. Ideas, please!