Sorry, I'm late. Here are a couple of questions you may consider, and why.
What size is your company ? IBM (Lotus) Notes + Domino is typically well suited for large or very large companies. It is robust, stable, 25 years proven track record and is unrivaled in distributed architectures, and is best served by a very specific set of technical expertise. However I've seen it used successfully in small companies.
What is the level of maturity of your company regarding security and privacy ? With out-of-the box two-factor authentication and built-in PKI, Domino is the most straightforward way to deploy an email solution that enables right away signing and encrypting e.mails. The slightly outlandish technical foundations also make it less prone to outside attack.
What about not using email ? Domino is super great when it comes to electronic documents management, structuring business processes (simple example : expense approval procedures, more complex example : quality control protocols in aeronautics manufacturing).
Also straight out of the box :
distributed, NoSql database management system
web server
PKI directory
LDAP server
full text indexer
integrated application development platform, all the way from rapid prototyping to industrial-grade, worldwide mission-critical deployment.
instant messaging
...
... all of which integrating swiftly and natively.
Let's face if : Domino is a bit like a giant combine harvester. If all you want to do is to mow your front lawn (i.e. move around a couple of smtp messages) it may be an oversized tool for the job. Now, if your company is willing to "think out the in-box" and do some smart knowledge managment and social stuff, the battle is not Domino vs Exchange, but Domino vs (Exchange + Sharepoint + SqlServer + VisualStudio + C#.net custom developments + IIS + Active Directory + Lynk + ...)
Now, the cultural bits to broaden our horizon : where it comes from, and what's next.
(sorry, selfish plug