I've converted all of my saved email to the ubiquitous mbox format and imported it into Mail.app. It is ridiculous. I have every email sent and received since April of 1999. I'm kind of pissed at myself for not beginning to archive sooner — I think I sent my first email either late 1993 or early 1994. The email I do have saved is from various clients including Pine, Mutt, Evolution, quite a few versions of Netscape (luckily, they have archived versions of every browser since 2.0 — this became really handy during the conversions), Mozilla, Outlook Express, and Outlook.
I didn't have too much trouble importing the mail files from Pine, Mutt, Evolution, Netscape, or Mozilla, as they all use the mbox format (note that the Linux and non-MS programs gave me no trouble). That isn't to say that there weren't any hang-ups though. I actually had to do quite a bit of work on a Windows machine to get everything converted. I found a great program for converting the OE .dbx files to mbox format, though on a few of the files it broke the From: field and so I had to find/replace across the entire file (not fun when the file size is 50MB).
The biggest problem was converting the Outlook .pst files. I think those had to be converted between three or four separate formats to get right. Let's see: installed Outlook on Windows machine --> imported the .pst files --> imported into Outlook Express from Outlook --> tried running the .dbx-to-mbx converter — it broke to all hell --> installed and ran Netscape v7.1 --> imported from Outlook --> tried bringing these files directly into Mail.app — no go — it would see the first message and nothing more --> installed Netscape v4.8 --> copied the mail files from the Netscape v7.1 directories into the corresponding Netscape v4.8 directories --> changed the filenames to match v4.8's naming convention --> opened v4.8 — it read the files fine --> moved those files to my PowerBook and imported into Mail.app.
Done! Now, in Mail.app, I have everything organized by year and sent/received since 1999.