Comment on Mail.app
As the keen reader will be aware, I took a big jump (given my large mail archives and many settings) and jumped recently from Eudora to Apple Mail.
Well, I'm still not happy with Search. I even bought RocketBox, but it is often confusing, and seems to often give missing results or just be buggy.
On the other hand, like I said it would be nice to take advantage, for once, of the extra functionalities that Apple ties in between the system and its own apps. One of them I just started noticing: using Quick Look for email attachments.
Given my job, photographers often send me ten, twenty or even more attachments in a mail. Photos. And in Eudora it was a bit of a struggle to view them, I had to scroll up and down quite carefully, and Eudora took its own time rendering them. But in Apple Mail I just click on the Quick Look button by the attachment icon, and then on the Full Screen icon (two arrows pointing to opposite corners), and I get a full screen slide show, fully controllable. Very nice.