I also love the ability to create tabs in a single window. They're easier to close when I want to return to the previously open tab than the multiple windows in IE or Netscape.
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I've experienced no difficulty with viewing any sites. It doesn't appear that incompatibilities are much of a problem. Either that, or website developers are getting savvier about not using browser-specific tags.
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There's also a new email viewer, called Thunderbird, that many are calling an "Outlook killer". I haven't tried it - I generally use my Yahoo account, or Popmail.
In general, the Open Source software that I've tried is functional and easy to use (Open Office, Linux, Firefox, etc.). I must caution, however, I don't generally push the envelope. I'm a lazy user - if the software does 80-90% of what the commercial product does, I'm satisfied with it. Minor differences don't bother me.