Not as a mail client, no. However, Mozilla Thunderbird is an excellent newsreader app in addition to being an email client. It easily does NNTP (if you're into newsgroups). And of course the nice thing about it is that it's free.
Probably not. In my experience, there is nothing you can do in Thunderbird that you can't do in Mail as an email client. I can't speak to the news features that chscag can but if you don't use it, there is little reason to use it over Mail in my opinion.
Tweaked up Thunderbird is much better than Mail. One example is you can send a email without that empty subject reminder, better choices of stationary , insert smilies , better spam filter, the list goes on. One click to insert animated smilies
The most recent Thunderbird has tabs which can be nice if you have more than one email open at the same time. It also doesn't have the image attachment problems that Mail has for Windows users.
Thunderbird lets you choose to save as attachment or embed it into the email. I still get Windows users that have problems with Mail's image attachments but it works fine in Thunderbird.
Also if you go back and forth between Mac and Windows then you don't have to get used to the differences between Mail and Outlook because Thunderbird is pretty much the same on both Mac and Windows. It's also a lot easier to share saved emails between Mac and Windows if you are using Thunderbird on both.
Also like Firefox there are add-on extensions and other customizations that can be done. If you are one of those people who like more customizations then Thunderbird has it. The average user probably won't notice much of a difference though.
Thunderbird is a nice email client but at least on my Power PC G5 Macs has always seemed bloated and slow. I much prefer Mail especially after 10.5 for the normal uses of an Email client. I can make Mail do exactly what I want it to do and more with the rules. I keep Thunderbird around just in case something goes wrong (Which can happen with any application over time) but use Mail for my day to day emailing. I have it set up with 5 accounts and never have had one issue.
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