Conversations in GMail are such an incredibly simple yet powerful way to look at mail. I get about twenty emails a day from various clients and I hate looking at a flat file of all emails, and I hate looking at "per client" email folders. With GMail I can just look at a specific conversation. I can see a few lines from each in the collapsed titles so that I can quickly remember what was said previously, and I can expand them if I need details or need to quote something.
But I can't justify using GMail to handle sensitive work emails, and we need to be able to access all my emails offline. So what do I use?