Wow. I was just looking for this today when I was thinking about Jeremy:
Roughly 60 days ago, I began My 30 Day Gmail and Yahoo! Mail Challenge. Obviously it lasted longer than 30 days, but the experience has been very, very useful. I've learned a lot about the strengths and weaknesses of both services.
Part of me wonders how many Gmail engineers have forced themselves to live off of Yahoo! Mail for a month. And I wonder how many Yahoo! Mail hackers spent a month exclusively on Gmail.
Anyway, here's the lowdown.