We are a large Exchange user - 10,000 accounts spread across multiple servers. Although our move from OS 9 to OS X has not been hampered by having a native Exchange client (or Exchange hooks in Entourage) the effect is worse - the Mac is becoming an extinct platform since it cannot achieve true parity with the PC community. Effectively, the lack of an Exchange client for OS X is killing off our Mac population so while some folks may view this as conspiratorial we see it as an unfortunate reality.
The Missing Piece: Microsoft Exchange for OS X