I have used Eudora for many years. Occasionally I try others, but find there are certain features that I miss. At work we are "forced" to use Mail which is ok. It has some nice things about it.

The pro's of Eudora: (1) ability to have multiple mailbox windows open (2) very nice search functions with flexible filter/quary options (3) task window gives useful feedback about status (4) handling of addresses is very simple (5) handling of sent mail is clear (Mail can cause messages to disappear somewhere between the draft & sent phase, so you don't know if your message ever existed) (6) after working with Eudora for so long, I can say that it works very smoothly, is quick and configurable in my daily communication flow.

The con's of Eudora: (1) as many have mentioned, it is not good at displaying certain HTML in messages (2) images need to be attached as files AND inserted into text or they don't display as expected (3) besides the fonts which you can freely choose, the "look" and style is a little stoic. And not that I need anything very flashy, but I think some improvements in the look and feedback (status message speed, etc.) would make it more friendly.