Lovely day flying from Wanaka over the alps and onto a small cruise ship.
A few things to be aware of:
1) Don't bother trying to decide between 'Wanaka Flightseeing' and 'Southern Alps Air' - it's the same firm behind the scene
2) Be aware that this is very weather dependent. My trip was postponed a day, even though the weather was fine in Wanaka it was no good in Milford. So if you only have a few days in the area book this as soon as you can. If getting to the sounds is a 'must' for you on your trip then don't rely on flying - instead take a land route to Te Anau and go to Doubtful sound - much much less weather dependent.
3) I had visited Doubtful sound on a previous trip. As a humble tourist, I could see very little difference between Doubtful sound and Milford. So if you've already been to Doubtful then probably not worth the extra time and money to go to Milford