Right now I'm using an (outdated) version of Mail For Exchange to push my work email to my N75. Before, I was pushing it to my HTC S730 (WM6 Std). S60 lacks the ability (in this version, anyway) for flagging, handling meeting invites, and moving messages to Inbox subfolders. That last I was unable to do successfully in Windows Mobile as well. The first two items bug me a bit, but not a huge amount...depends on what's important to you.
This version also does not have access to the Global Address List, but I know they fixed that in later versions (which won't install on my N75).