I've always used Postfix as my SMTP server, then I typically bolt on Courier to handle POP and IMAP functions.. everything works rather well setup in this fashion.. I've never really considered trying to run just Courier..
Check for some comparative reviews..
http://www.geocities.com/mailsoftware42/ (make sure to look at the subjective charachteristics in this one.. )
http://shearer.org/MTA_Comparison - no Courier as MTA in this review..
If you want a very comprehensive server with minimal configuration take a look at
Zimbra.. what a nice piece of work. install a couple packages, configure through a web interface. ajax web client. very nice.