I compile it in when I build from source, but frankly I haven't used it really that much. More of a "it's there when I need it" situation.
Kind of a moot point tho, on my PHP5 server, it's not there as a separate entity, mostly because I only use one switch (iirc) that controls any common XML extension:
--with-libxml-dir=/usr/local
as opposed to the following in 4.3.4:
--with-xml \
--enable-expat-dir=/usr/local \
--with-dom=/usr/local
My $0.03 😃