It's happened a couple of times since, and each time is as infuriating as the last.
What pushed me over the edge, though, was Mail's stubborn refusal to subscribe to the shared folders at my work. Depending on the value chosen for the IMAP Path Prefix, I either got all of the shared folders, or none. I had little choice but to opt for the former, but as the company archives have grown, this has meant storing about 60GB worth of messages under
~/Library/Mail. With various other demands on my meagre old 128GB disk, I decided I'd had enough (my upcoming MacBook Pro will only have a 256GB SSD; I couldn't justify the cost of the 512GB one).
So, years after I gleefully abandoned it, I've reluctantly returned to Mozilla Thunderbird, now at version 3.1. It's improved a lot since version 1, but it's still pretty hateful. I'm compiling a proper list of Pros & Cons, but I don't know if I'll ever be able to cope with the sheer ugliness of it. Take this dialog, for example:
I can only hope that the version of Mail included with Lion will be a suitable replacement.