As far as I can tell, at least in the SQL Server world, UDT aren't used very much.
Trouble with UDT is the fact you can't easily update them. Once created and used in databases, they're almost like set in stone.
There's no "CREATE OR ALTER (UDT)" command :-( So to change something, you have to do a lot of shuffling around - possibly copying away existing data, then dropping lots of columns from other tables, then dropping your UDT, re-creating it with the new structure and reapplying the data and everything.
That's just too much hassle - and you know : there will be change!
Right now, in SQL Server land, UDT are just a nice idea - but really badly implemented. I wouldn't recommend using them extensively.
Marc