I have been running Lucid since Alpha 1 and it is now stable enough for me to
use every day. I still have my Karmic installation, but have not been into it
for a week or so.

I plan on doing an upgrade with RC1 on my Karmic partition and then fresh
installation once the final is out (a couple of weeks later actually due to
inevitable slow servers), but there is no need to. It is just my established
practice. My Lucid installation is on my experimental partition which will
become 10.10 when it comes out in the summer. This is my way of doing things.

There are many reasons to do a fresh installation. There are always residual
traces of the previous one that can make your system not a good as if you do a
clean installation. It makes troubleshooting a lot easier, for one thing.

There are equally compelling reasons to not do one such as if you have lots of
packages that you don't want to install afterwards or have bandwidth
restrictions. If you find installing worrisome then it may be a reason to keep
it as it is.

In either case you should use a cleaner to remove old kernels and package
caches.

For Karmic users, there are lots of improvements aside from the look and feel.
The performance is great. There will also be the Me Go Menu for social
networking and the Ubuntu One music store in Rhythmbox. KDE users get KDE 4.4
with lots of improvements. You can now have icons on the desktop without the
folder view plasma and a different wallpaper or activity on separate desktops.
Choosing widgets or plasmas is now drag and drop. However, as always if you are
happy with what you have then upgrading can be more of a headache than it is
worth. Nvidia users need to be especially wary. The default driver will be
Nouveau which is so far only 2D (no desktop effects or compositing). You need to
use the proprietary ones which work fine and give 3D. No big deal, but it is
just another hoop to jump through.

Kudos to Canonical for their implementation of Plymouth with Nvidia cards. I had
to live without 3D for a few weeks (Alpha 1 & 2), but then it just worked when
Alpha 3 came out. BTW, Nouveau will eventually give 3D. They are working on it
with some success. It should be available this spring. (Fedora in contrast does
not provide any help. It is up to the community to provide solutions. Fedora
gives you Nouveau and that is it. Getting 3D to work in Fedora is not for the
faint of heart.)