so if they are not a replacement for MS Outlook, then which open source clients are a good replacement for MS Outlook?
That depends on what you want out of MS Outlook and how many of those enterprises features do you really use.
If you only want email and are happy accessing them over imap/pop3, any of the open source clients will do the job. If you want to access email over MAPI, which is the protocol MS Outlook uses, evolution has a MAPI plugin, which is quite new and still a little buggy, but it works. Evolution also nicely integrates with the gnome desktop and has a decent calender application.
If you want features of outlook like delegation and other advanced features, IMO, there is nothing like that which is open source. I might be wrong, though.
The thing with Outlook is that it integrates tightly with Exchange server and uses proprietary protocols to do so, which the open source world doesn't have access to. If you remove Exchange server from the picture and use something like zimbra, instead, you'd have similar functionality in the open source world too.
-- Sharninder http://geekyninja.com