Sometimes it simply hangs sometimes it won't even close and no one, even in Redmond, can probably tell you precisely why. But its competitors do a fraction of what Outlook does. There are calendars and schedulers but without email. There is email, but without remote access or fax. There is contact management, but without programmability or grouping or categories.
Outlook does it all and that it works ever is a minor miracle.
And for all of its problems it is great software. From one location you can send email, sort contacts by categories and schedule meetings with shared calendars. Oh, yes, to do all of this you must currently be on a Microsoft network. If you go to a competitor, all of these marvelous capabilities are no longer available. And they basically work because Microsoft provides either Windows or NT for the user.