My mail files were at 2GB and growing, and I had to copy and transfer my 2GB (and growing) User database everytime I went home. With the introduction of NUS LargeMail, I can switch to using the IMAP server. To access mail on IMAP, I can use Mail.app interchangeably with Entourage, and even Thunderbird if I feel like it.
Additionally, if I am referring to my Archives in Entourage (I have relevant mail dating back to 1998), I can have Mail.app open to my current inbox. All I have to do is cut and paste, or email relevant portions out. ANd I can enjoy the benefits of Mail's better integration.
But OS X's Mail.app still needs some help before it's up to speed with Entourage - and there is plenty of help out there; e.g. Hawk Wings suggesting a way to speed up Mail - hawkwings.net is full of the good stuff! There are also plugins and descriptions at tikouka.net and maczealots.com.
Spam Sieve - I've been using SpamSieve since Feb 2004 and its great - now it catches what NUS' filter lets through (all my other accounts are redirected to GMail). See instructions on the webpage.
Letterbox 0.16 (three-paned view) - I installed Letterbox on the office G4 this morning, and it's lovely. It was MisNomer's recent post that prompted me to revisit this, and all it took was a couple of seconds and it now looks lovely!