Lotus Domino R5 vs Alternatives

In searching of information, I have found very little on the comparison of Domino R5 used as both a progressive messaging infrastructure as well as a best of breed web/browser based application development environment. As a CTO for a major company, this does intrigue me. Many clients request comparative strategic information to reinforce their infrastructure decision. I would very much welcome the thoughts, ideas, strategic thinking or experiences of any other person with regards to this topic ?