CEGUI is also a good choice and fairly stable and i believe it has a decent feature list.
last i checked they were developing their own gui designer to create your gui's in.
( i know they had an older designer, but they were creating a new up-to-date one http://www.cegui.org.uk/phpBB2/viewforu ... 52e092ce64)
they also have an irrlicht renderer built in as well as tutorials.
worth a look...