Thunderbird is written by Mozzila. The same people that make the Firefox Web Browser.
When I switched over from Windows XP to Windows 7 there was no more Outlook Express and I had to use Windows Live Mail, I think it was called. I didn't really care for it so I tried MS Outlook as I have MS Office in many different versions that all come with Outlook, didn't care that much for Outlook either. So I searched around a little and hit on Thunderbird. Installed in on my Win 7 desktop and liked it. Fairly easy to setup and use. As with everything there is a learning curve to getting use to it, but not much.
When I bought my Macbook Pro I tried Mac Mail for maybe a day and then downloaded Thunderbird for OS X. Installed it and have been using it on both PC and Mac. I would never go back to Mac Mail.
There are also many add-ons for TBird. Right now I'm used CompactHeader (Give you the option to show or hide the full header of the emails), Extra Folder Columns (Adds some columns for different things), Folderpane Tools (That allows you to move your email accounts around in the main window. Otherwise they are listed as you add them and I have 7 Email accounts), Get All Mails (Which give you a button to Check all your POP Email accounts at once. You can do that anyway but you have to select the drop down from the standard Get Mail button), Google Contacts (Which syncs all your Google contacts into the TBird address book), Keyconfig (To set shortcut keys differently then the default) and Lightning & Provider for Google Calendar (Only on the Windows install of TBird. That is to sync my Google calendar into TBird. On Mac I use the built in Calendar program).
I've been using it on Windows for almost 2 years and on my Mac from about the second day I had it, which is about 15 months ago.
Most setting are already included in the program. You just type in your email address and password and TBird either has the settings or goes out to the server to get the correct settings. The only account I have to set up manually is my companies email account. We use some Fly By Night Hosting company, Name Withheld, and although they do IMAP no email program has or can get all the correct settings for it. Other then that one account all others, well until AT&T started to move away from Yahoo and started up there own Email server service again, are done automatically. Like with Gmail. I type in my gamil email account and all the settings are filled in and they WORK, and have not STOPPED working.
I have had Zero Problems with Tbird.