I have a Windows Phone, my brother also owns a MS Band and he has an iPhone (not the latest, but the one before), another family member has an Android phone (Galaxy something). The MS Band app is available on all three stores but only mine integrates with Cortana (which is great).

To get the Cortana integration, you need to have a Lumia (Windows Phone). Which is understandable, since Cortana is proprietary to the Windows ecosystem, just like Siri is to iOS and Google Now is to Android.

The rest of the Band features work on either Android or iOS. The New Microsoft seems to be platform agnostic where it makes sense. Which is cool.

I've got a Lumia 920. The software is excellent, but the design of the case could be better. I added skateboard tape to the back to keep it from slipping out of my pocket (which always seemed to happen in movie theaters...) Resale value won't be anything like that of an iPhone, mostly because the plastic case is showing wear & dings that the glass + aluminum iPhone wouldn't have.

I also have a Band. It doesn't work with my iPhone 4, just 4S and above--although maybe this update will fix that (unless the 4's bluetooth is too old).

I use it with the desktop sync app. This means I don't get text notifications or anything like that, but the health tracking is fantastic.