I've successfully upgraded my primary web/mail/misc host from Debian Etch to Debian Lenny. There were a few minor problems, but on the whole the upgrade was as painless as I've come to expect.
In the past I'd edited my Exim4 configuration to add quite a few ACL checks, for example rejecting mails based upon spoofed/bogus HELO identifiers, and rejecting messages that didn't contain "Subject" or "Date" headers.
The Debian Exim4 configuration may be split into multiple files (which is how I prefer it on the whole). The idea that you just add new files into the existing hierarchy and they'll magically appear in the correct location when a real configuration file is generated. On the whole this works well, but sometimes editing files in-place is required, and it was these local edits that caused me pain.
Fixing things up was mostly not a challenge, primarily it was a matter of removing ACLs until exim4 started without errors - all my spam checking is handled ahead of exim4 these days, except for the last-ditch spam filtering with a combination of procmail-fu and the crm114 classifier package.
Taking a hint from Bubulle's weblog I decided to nuke my CRM114 spam database to avoid any possible version-mismatch issues so now I'm having to classify a lot of "unsure" messages. Happily my memory of doing this last time round is that the initial training of spam/ham takes a day or so to complete.
Anyway now I can start looking to advantage of the things new to Lenny. But probably not until I'm sure things have calmed down and upgraded correctly.
steve@skx:~$ uptime 05:00:31 up 260 days, 14:23, 2 users, load average: 0.95, 0.51, 0.31 steve@skx:~$ cat /etc/issue Debian GNU/Linux 5.0 \n \l
ObFilm: Bill & Ted's Excellent Adventure