Mono vs. Stereo

I've been wondering for some time what the sound difference and listening experience is between Mono and Stereo.  Let me acknowledge first off that I have not done an A-B comparison since I do not have both Mono and Stereo of the same release.  At least I don't think I do.  Would appreciate a couple of thoughts on the difference.  If someone has a good link to a source write-up, that would be appreciated as well.  

I've heard folks say the mono versions are better, but again, never did the A-B comparison.  

Thanks in advance.