I disagree. Here's why:

Exchange means they're using the Exchange calendar, for which the only clients are Outlook, Entourage, and OWA (which is bad at the best of times, but which is awful if you're not using IE; for example, without IE, there's no email search feature).

Exchange automatically converts every text/plain email into HTML to present to IMAP users. This might be okay if it would bother to use a monospaced font.

Exchange munges RFC(2)822 emails by separating incoming mail headers from the body and reconstructing the RFC(2)822 email when needed, for example, for IMAP clients. The resultant email will look sort of like the original, but will be different. In a particularly egregious flaw, it throws away all "Resent-" headers except Resent-Mailer.

Exchange means most everyone is using Outlook. Outlook has any number of bugs, like splitting a long URL at 72 (or some other arbitrary number of) columns, without using quoted-printable's line continuation feature, despite setting Content-Transfer-Encoding to quoted-printable. In order to resolve this, Outlook would have to take the extreme tactic of adding a "=" to the broken ends. (Outlook, of course, knows about this Outlook bug and works around it, which can break correctly formatted messages with long URLs by combining them with the next line.) Amazingly, it seems no one has written a Thunderbird extension to emulate Outlook's workaround. It could work even better by only applying it to messages that have the header "User-Agent: .*Outlook". Oh, except Outlook doesn't bother to send a User-Agent header.

Oh, and Outlook automatically expands email groups into a list of everyone in the group, so there's no way to have your mail client tell you if a message from Outlook was sent directly to you or to a group you happen to be in. And I always feel like a prick doing a reply-all to a list of fifty people instead of just a reply-to-group.

Outlook also automatically converts emoticons to webdings. Of course, a smiley-face in webdings is encoded as "J", so it sends a "J", but it encodes it as us-ascii, which means that any correct email client will just display a "J".

Outlook means that if someone sends you an email that contains nothing but ":)", you will receive a 2300-character HTML blob (I counted. Okay, wc counted.) that tells you "J".

Outlook also means that everyone will be top-posting.

Outlook means forwarded messages will be in that horrible Outlook forwarding format where it's inlined without any quoting indicators, and the few headers that are included have actual email addresses stripped from them. And there's not any way to turn on any other method of forwarding. Not that the Frankenmail that Exchange generated is going to help you out anyway.

And if your IT department rolled the critical failure of Exchange plus Blackberry Enterprise Server, have fun watching every email sent to you get delivered, deleted, delivered, deleted, delivered, deleted, and delivered again, all within a few seconds. I'm pretty sure each of these has a different IMAP UID. Sometimes it'll do it again a few minutes later, leaving behind such unimportant flags as "read" and "responded" while in limbo. And if it decides to delete it while you're looking at it in Thunderbird, all sorts of fun things can happen until you reselect the new version, like it can't find the attachments, or if you forward it, it's empty. (Is this a Thunderbird bug? Yeah, maybe, but it's still egregious on Exchange's or BES's part.)

Also, IT departments are loath to support multiple clients (with reason). So when you encounter an IMAP-related problem, they're most likely going to tell you "too bad; use Outlook". This is fun when the Exchange server is rebooted and decides that it didn't need to start the IMAP server this time.

Other than that last one (and then only because I am the IT department), this is all common, recent personal experience. And I'm pretty sure there are more that aren't popping to mind. There are other serious complaints and whines.