For someone like me, a self taught coder, SQL has always been a hurdle. I am sure if I dedicated some time (probably quite a bit) I could get good with it, but I don't really want to in all honesty. I have built some fairly complex sites to date including e-commerce, community and download sites, but have rarely had to delve into SQL directly. The query builder lets me build relatively complex queries with only a very basic working knowledge of SQL.
So personally, I love the CI query builder. For me it is one of the really great core aspects of CI.
But I suppose that just puts me firmly in the camp of "it's nice", but 'nice' doesn't seem to give it the importance I think it deserves.
Paul.
PS I have never used anything other than MySql so this opinion is nothing more than a non-expert's opinion with very limited knowledge and experience.