I feel for zirconium3 and tylervo.
Our conglomerate companies (one IT dept for all) are split over the usage. The two large companies use Exchange, the two smaller companies use Notes/Domino 5.
The problem for us really stems from the controlling hand of the business execs making the decisions without understanding the tools. So we received Lotus with zero training for the IT staff. We all learned by books and hard knocks. 3 years into Domino now and we three administrators are just now getting a grasp of the full potential. Users got even less. The execs wanted to throw Notes their way with zero training. As a result, users just barely made it work. I finally got to train about 400 of them on basics with a few bells and whistles, but the company still thinks of it as a simple email system with a few databases attached.
Bottom line opinion: if the company will not take the efforts to learn Notes/Domino well, go with Exchange. It is clear they only want a simple e-mail program. Of course you'll be vulnerable to every virus out there.
If the company has vision and will train to back it up, go with Notes/Domino. Version 6 is better than ever, and you can build so much infrastructure with Notes. My previous company knew Notes 4.6 intimately, and had databases for timecards and personal information bulletin boards with very easy instructions for users to follow. Users were also given much more instructions empowering them to create useful materials.
One interesting point about Lotus is that we are less susceptible to the "knuckleheaded" user opening a virus. Used to be when they did that on Exchange, it would grab outlook or Exchange client and replicate itself. Now if the user opens the virus, only their box is infected because most viruses out there don't seem to be written to use Notes to replicate. Of course there is no substitute for AV software, but its nice to know if the AV fails for some reason that we are less vulnerable.