Well, just off the cuff... look at this:
http://www.innovationcreators.com/2006/08/lotus_notes_email_vs_microsoft.html
I consulted at a company using Outlook last year and had to wave my Lotus Notes email goodbye for six months. It is not a bad product, but nothing to write home about. I couldn't wait to get rid of it, but I guess if I had to live with it, I would. But please note that it is an email/calendar/address book application, and that is where it stops. The whole database/application development combo architecture that Notes offer can not be beaten by Outlook.
With regards to user experience I have found that you love what you know and are used to. So, if you have exisitng Outlook users in your company, chances are that they will resist the Notes interface. For them, I suggest you have lots of patience and training showing cool and useful features OR you consider deploying Domino Access for Microsoft Outlook (DAMO), which means they keep the interface but you can have Domino in the backend.
Total cost of ownership is lower for Domino. The End. I don't even want to debate that - I have just witnessed it at so many clients, especially with regards to hardware requirements and number of administrators. Just make sure you employ one excellent administrator, it makes all the difference.
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