For anyone who doubts the worth of testing new designs on their website or emails, take a look at this test from “Which Test Won”
I'm probably givingthe game away when I admit that:
A) I got it wrong
B) So did the majority of other people – most of whom are marketing “pro's” like myself.
Marketing is at the end of the day a social science – dealing with humans, and humans are diverse and unpredictable.
Sometimes (quite often in fact), tweaks to your website which seem obvious sat in your marketing agency's office, turn out to be a disaster when your “real world” users get their hands on it.
Test, test, and re-test, everything you do. You might not improve your results with every change, but you'll have the confidence that what you're doing isn't sending your results downhill either.