The last PIM that I looked at was SOHO Organizer (CHRONOS). If you need something that works with Exchange (without fighting with your IT group over IMAP), then Entourage is the only show in town. Google also found, the ChandlerProject . . . although it seems like you would use this instead of using OF.
If you ignore calendaring, other apps to consider are (linking Macworld reviews that are now a bit dated):
The previously mentioned Yojimbo
I use DEVONthink Pro Office, which allows me to import email in bulk from Mail. This is a feature that sets DTPO apart from the competition, but it does not understand discussion threads.
The nature of DTPO's document indexing means that you do not have to deal with tagging, but you still have to setup and populate folders to hold your documents. Once you seed your folders with representative documents, DTPO will auto-file your documents as you import them. However, auto-filing works better if you are willing to build a detailed set of folders for storing your data (you can do this as you go) and if you are willing to chop up multi-topic documents into pieces. This is a product that does well with large amounts of data.