... Entourage's database self destructs without warning when it hits a gig, and ppl around here like to keep hundreds of 20-50mb PDFs in subfolders "just in case they need them later".
My employer recently switched to Outlook/Entourage and has been in close contact with Microsoft due to some issues with the calendar and email tools. Microsoft is paying attention to us because we include one of the large government centers and they want our experience to go well. We have been told that there is no file size limit with the database (mine is over 5 Gig so far; I've seen others much larger) as long as the OS can handle the file. We were also told that 2 million messages is the limit. This seems large but it is misleading because each email counts as two and every calendar entry counts also plus there are multiple emails that are generated for each calendar item. And our secretaries sometimes manage calendars for a dozen people or more, so multiply these things all up, and 2 million doesn't look that big. If you just use email, I think 2 million should be fine, and subsequent versions of the program will allow for even more. (We've had Microsoft engineering teams come visit us to discuss these things.)
I use Outlook (on Windows via Parallels) and also Entourage (the Mac client) and at this time, the Mac Entourage client is much more limited and has many more "issues" (bugs) than the PC client. A new version of Entourage will be out soon that will address some, but not all, of those issues.
The Microsoft Database Utility (included with MS-Office) can be used to rebuild the database for Entourage, which will solve some problems.
Also -- Entourage 2008 is more reliable than 2004. In addition, use of both 2004 and 2008 on an Exchange Server will definitely cause problems, many of which will go away if all the users update to 2008.
