This is a great day out in Auckland especially for family/friend groups.
I would recommend buying your ferry tickets online to avoid the hassle of having to buy them prior to boarding or avoiding missing out (especially peak times like school holidays, weekends, summer) on the ferry time you want, although there are several times per day.
Excellent views at the top. Walk straight to the summit or do other walks around the volcano.
Summit walk is 1 hour to the top, we took less time coming back down.
Take food and water (no shops), only toilets available. Take a jacket or be prepared for the weather/season you go in.
Cell phone reception available. I am with Vodafone and it worked fine.
Cafe available on the ferry if needed.