Can highly recommend doing Milford Sound using Mitre Peak.
Firstly we did it on a day when it rained quite a bit and it was great; do not be put off if the forecast does not look great, we weren't and did not regret it.
Of course the sound is amazing to look at in any weather, with the waterfalls being spectacular in full flood. The Mitre Peak on-board commentary was excellent; just the right amount of detail and informative, and never intrusive, so you had time to enjoy what your were looking at. The staff on-board were great and friendly, and the on-tap complementary tea/coffee was much appreciated.
Mitre-Peak sell themselves on a personal approach, with a small boat which can get closer to the waterfalls, wildlife, etc. This is true and really does work.
This review is the subjective opinion of a TravelPod member and not of TravelPod.com.