I had huge perf problems with my 3G after installing iOS 4, and no update ever fixed it. However, it is relatively easy to fix, just a small bit of pain. To fix this, connect your phone to iTunes, then restore, but don't restore a backup... Do a new install. You then have to reinstall your apps/photos/music/etc., and reset all your preferences, but after doing that, my iPhone is basically back to where it was perf-wise with iOS 3.x, and I get the new features of 4.x (which I don't really care about). As others have said, it's very hard to go back to 3.x because they upgrade the baseband firmware in ways that are apparently incompatible with OS version 3.x.

I've been using TMDA (http://www.tmda.net/) for over 2 years. During that time, I've received only 2 spam emails in my inbox, that were due to setup problems. My email addresses collectively receive about 100-200 spams a day, but TMDA does its job, so I never see them.

For those who don't know, TMDA is a challenge-response based server-side system. It's open-source, all written in Python. Works with all client mail readers. Check it out