this post is a comparison of the various email clients available on mac osx
imap, pop, exchange, gmail, hotmail, yahoo mail
price: included in osx for free
major bonus: all emails are included in your spotlight searches
get it here: http://www.sparrowmailapp.com/mac.php
gmail, imap, pop
price:£9.99 (with a free version thats ad supported)
sparrow has some cool features, like facebook integration to show nice pictures of your contacts in your inbox, but osx has this built in too, so the same things are available from mail.app
imap, pop, exchange
price: free with an expensive copy of microsoft office
there isnt really any one thats better than the others, they all do email.
At work, I use thunderbird (with the excellent thunderbird conversations plugin)
I think if you NEED exchange, then you are stuck with mail.app or outlook, but even exchange servers support IMAP, so if you are nice to the administrator (or if you ARE the administrator) then its probably best to try them all out.
also, if you NEED your emails to be searchable from spotlight, then mail.app is the one to use.
for now I’m sticking with mail.app