Personally, I haven't found Thunderbird to be lightyears beyond Outlook, so I don't emphatically recommend it (I'm sure others do.)

However, Firefox is a masterpiece, and it's free - plenty reason to encourage friends and co-workers who don't generally like change to try something new. And in almost all cases, they have agreed Firefox is better and they can't imagine going back to IE.

Firefox is so good, in fact, that Microsoft has been forced to improve their own product. Competition (and encouraging competition) is always a good thing.

Why haven't I encouraged people to try Opera, etc? Because those other products either aren't free or aren't as functional. IE is preferable to those products in many ways.

Bottom line: I like sharing good things with people I know, and, frankly, I like the thought of Microsoft getting scared and having to actually compete.