We've recently migrated to Microsoft Exchange. My impressions of it are that there are some good points to it, but the bad ones are a real pain in the neck, and generally translate to higher cost of ownership than other solutions.
We have all the latest stuff here. We bought a (dedicated) HP Proliant DL380 server, Windows Server 2003 and the latest Exchange. We had it all installed by a certified consultant, who's done a really nice installation job. Also, we got all the clients (all 20 odd of them) onto Outlook 2003. So, everything's about as "good" as it can be.
We have an anti-spam service from MessageLabs. It's really good, and gets just about all the actual spam. We've got it setup to mark spam, but not to ever reject any. So, we should be able to get clients to figure out what's spam and what's not and display accordingly.
Now, on Lotus Notes (which is far from excellent), we had a agent running on the server which would automatically move all spam into a spam folder in everyone's mailbox. In other words, it's server controlled. One change, everyone gets it.
In Exchange, this doesn't seem to be possible. Instead, each user has to manually add an anti-spam rule in their copy of Outlook. If two users shared a machine, they'd both have to do this. There's no (obvious) way to roll it out corporately. Also, any new users need to do this when they start. Again, there's no way to "build" the rule into a Ghost image or something. In other words, the cost of anti-spam in Outlook is higher than it needs to be. It's certainly higher than Lotus Notes.
Take another example - the "message sending format". If you send a Rich Text Format email to a non-outlook user (heaven forbid such a thing should exist ;-) then that user only gets the message body (in plain text with no formatting) and cannot access any attachments (because they're all buried into a winmail.dat file which only Outlook will open).
You can avoid this enormous incompatability issue by having Outlook users send in HTML (or plain text). Just about every email program in the world can read one of both of those formats, so it's got to be the way to go. However, again, there's no server side way to control this (that I could find). If the client sends in some Rich Text, then it goes out in that format. Once again, every client has to ensure they've got the right settings. Again, there's no way to build this into a Ghost image or something, so every user has to set it for themselves. Again, higher cost of ownership.
Once again, in Lotus Notes this was dead easy - it's a server setting (and besides, RTF doesn't exist in Notes, so it's only ever going to send plain text or HTML).
I know full well that Lotus Notes is far from the 'silver bullet' here. I know it's sometimes very complicated for users to use. The fine grained permissions and the like can cause problems (especially if semi-knowledgable users fiddle with them). Actually, out of the box, Lotus Notes (even V6) doesn't seem to have much for anti-spam, but it's pretty easy to add a couple of views and the agent which you can download from the Internet.
Outlook isn't really a good idea for a lot of thing either (Lotus Notes can be a pain, so maybe don't use that either ;-). Outlook encourages really bad habits. By subtle design choices, it encourages "data dumping" - that is, you can easily end up keeping every single email you've ever received or sent, every attachment still in place, and you end up with your entire working life contained in your inbox.
From a company point of view, this is a terrible way to operate. If attachments and the like arn't being filed properly somewhere, then they offer no benefit to anyone except the current owner. Indeed, if that person forgets they've even got that attachment, then the storage of that data is only a cost to the business, and brings absolutely zero benefit.
A far better way to work requires a bit of self discipiline. From an Outlook perspective, you have to file your attachments on a server someplace (in a properly organised hierarchy of folders). You have to delete all the "are you coming to the pub tonight" type emails. You also have to think about clearing out unnecessary stuff in your 'sent items' folder. Also, you need to think about organising your mails into folders and having a suitably well organised archive. You need to be wary of auto-archive, because this will move all those "are you going to the pub" emails, which of course have no value to anyone after about a day of receiving them.
All this is a pain in the neck. I know no one does this stuff, which is why PIM applications should be providing ways to do this with ease. I know few other PIMs are doing it, but Outlook is somehow the worst.
The Outlook/Exchange combination will cost you more than alternative solutions. Choose carefully - it's not always that obvious how something will operate before you adopt it full time.