Some answers;

You can have several calendars, at least if you have several accounts that can be contacted with individual password and account-name. I haven't found any way to add shared mailboxes to my own account's settings though.

Folders support is good. Includes nested ones.

There is quite a lot of different extra fields, but they stay empty, at least if you use search for contacts from Exchange's address book. The ones that get synced and that are specific to my account (own contacts that can seen from Outlook) have more information in them.

There is no better Exchange client for N9 as there are no others. Compared to default Android Exchange client or even Windows Phone one, I prefer N9 one because of a very good calendar client (and that company directory search works too, unlike in Android).

I haven't tested client called Touchdown on Android though. You may want to check that before making any decisions... I have no experience on iPhone email client either.