The answer is yes it is better if you are talking Ubuntu12.04 vs.
Ubuntu11.10. The Unity desktop seems to work better than older versions
as much work has been done on it.

Regarding my postponing the installation of the 12.04 to replace 11.10; are my
fears unfounded about messing something up?? Any advice before I press the
button?? I think I temporarily lost my mail (quickly returned), when I hit
"Quick Filter"; was that likely?? and what is the purpose of that key??

OS upgrades can go wrong so it's always worth being prepared for it,
at the very least vital data should be backed up onto an external
source ( HD, Flash Drive, CD/DVD ). Also if you've upgraded already to
11.10 from an earlier version the chances of it going awry multiply.

In general it's the better idea to always do clean installs when
moving up to a new version of any OS