I can only speak from personal experience, but I upgraded
from Mint 7 to Mint 8 main using this guidehttp://www.linuxmint.com/blog/?p=1144
and everything went perfectly.I used the GUI method.
I backed up my home directory first just in case.
Follow the instructions to the letter and you should be fine.
It does not upgrade you to Grub2 so your existing Grub
will be intact after the upgrade, it will just
have Helena instead of Gloria.
I initially was not going to upgrade and was going
to wait for the next LTS but an update to Gloria caused
me stability issues that disappeared after the
upgrade to Mint 8.
All your settings and data will remain intact.
Things are more like they are now than they ever were before.
Dwight D. Eisenhower (1890 - 1969)