I've only heard Ah-dah/duh-line. I've only met one little baby girl named this, and that is how they pronounced it.
I think it is certainly less common than
Madeline, but it is definitely in the same vein of
Madeline,
Madison,
Addison, type of names that will almost inevitably be shortened to
Maddie &
Addie. Sorry, but I'm so sick of hearing
Maddie &
Addie. I'd much rather here
Madeline &
Adeline. :)