IBM are on record as saying that their version (the U2 databases) is the fastest growing product in their database VAR sales.
And International Spectrum is holding their conference right now - against a background of collapsing conference attendances (typically down 25 - 50 %), they're holding their own - I think they were down 7% (or was it up?)
imho relational *theory* is great. Unfortunately, relational practice falls foul of Einstein's corollary to Occam - practice is TOO simple, therefore system complexity (as in all the stuff *round* the database) rises sharply as a result. SQL queries are a classic example :-)