I love free software, but I don't mind spending a bit of money on software that truly fulfills its pupose. Such a piece of software is Touchdown, by NitroDesk, probably the best MS Exchange Client there is for the Android.
In fact, I think that the software is so good, it easily compares it's features with the likes of Blackberry or Windows mobile for Exchange connectivity.
I know that you could fetch Exchange email via IMAP (if your system administrator allows it) or even via the email application included in Android these days, but there are reasons why you wouldn't want to do this.
My Android is my private phone, but I also use it for work. I do like to have an office and a private mode. I don't want my appointments to appear in each other's diary all of a sudden. I also want to keep my phone-book and that of works separate and you might have tasks and other features that just simply don't synchronise properly over Android's native software.
Touchdown has no problem with anything. See HTML emails and reply in HTML, you even see the picture of the person that you are corresponding with (provided you have the pictures available on your Outlook).
Another important feature for me is booking meeting rooms and scheduling meetings while not in front of the computer. It all works very easy and is quite intuitive with NitroDesk's Touchdown for Outlook.
The beauty of the product is that it's completely customisable: choose how much of each email you want to download, choose your in office / out of office times, set signatures and how many days to synchronise. It syncs emails, calendars, contacts, tasks and notes - the complete package.
You can even search through your inbox or any folder that you have synchronised with.
It's a great app to have!
Get it from the Android market.