I have been back and forth with numerous mail apps. I used Apple Mail (I), I used Thunderbird, I used Entourage 2004, I used Apple Mail (II - Leopard version) and I've used the web interfaces for Gmail for a time exclusively. There are probably others but memory fades...
Anyway, I got Office '08 and went back to Entourage. Sad fact of the matter, for all the M$ hate, this is one application that does everything (Contact Managment, Email, Calendar, etc...) inclusive and seamlessly.
The way OS X does it - separate applications for each function is certainly more aesthetically pleasing but it seems like it: 1) Goes out of it's way to demonstrate the simplicity of the apps which 2) Makes them feel less like "professional applications" and more like something I would have used in college or something for the hipster housewife (no offense to hipster housewives, I have one myself).
While I am no fan of M$, if you have to work in the business environment, or on the fringes like I do, Entourage just works better IMHO.
Dogfood1