We went over Christmas a couple of years ago - went from Melbourne to Sydney, then across and down the far side of New Zealand, stopping at about 6 different towns along the way I think, then Hobart, then back to Melbourne. It was 21 nights. We got an absoiute bargain - less than $1000 for each of us, and an excellent all inclusive drinks package - the likes of which I have never seen advertised since! We booked through an American company online, Vacationstogo, and the aussie dollar was a very strong so that helped.
The cruise ship we were on was part of the Costa Group - the same one that had that disaster in Europe (which may explain the price!) and it was at the end of a cruise all through Europe, down through Asia, then around Aus and NZ. As such it had a lot of europeans on board, and everything was translated into four different languages. The entertainment wasn't all that flash, but we drank lots and met a really fantastic couple that we spent most evenings and days out with, so we weren't too fussed about that. We're not huge "joiners in" anyway! Food was brilliant. Onshore trips - we organised stuff to do in each of the ports ourselves, such as car hire in one place, bike hire in another, a sailing trip in another. Much cheaper doing it yourselves than through the ship-board bookings. NZ was lovely, and it was a great taster, to decide which places we'd like to go back to for longer in the future.
The ship sailed mainly at night but it was a little boring when it was at sea - I think three days was the longest stint. As it was Christmas/New Year, the weather wasn't all that flash for lazing around the pool, but as I said - excellent onboard drinks package helped us through!
I'd do it again if a cheap enough offer came up - OH was a bit bored as he's a lot more active type than me - there was a gym on board and some (sad) people did the 'round ship jogging/walking course' each morning! I did a free "Conversational Italian' course on board which was fun.
I don't think I could ever bring myself to pay full price for a cruise though - there are lots of discount sites and special offers it's worth keeping an eye on.