I didn't do the indoor exhibit, but I will next time.
Definitely a "must do" if you're in Wellington, especially if they have kakapo! I'm guessing the night tour is also worthwhile (but something else I didn't do).
the indoor exhibitry is very good and well worth including in a visit (it costs extra). The night tours give you a good chance of seeing little spotted kiwi, but they are expensive.
The sanctuary is definitely a must-visit if in the Wellington area, especially as a number of the birds that live there cannot easily be seen elsewhere on the mainland.