Let's assume you upgrade from package/foo-1.0 to package/foo-1.1. You can then use quickpkg to create a backup of foo-1.0, upgrade with ''emerge -ua foo'', and test it. If you are not satisfied, mask the updated version in /etc/portage/package.mask, and ''emerge -uak foo'' again in order to downgrade. -k tells emerge to use the binary package built with quickpkg instead of re-compiling. 

<code bash>
# Make binary package
quickpkg foo # (or quickpkg package/foo-1.0) 

# Upgrade foo
emerge -ua foo 

# Make new version
echo ">=package/foo-1.1" >> /etc/portage/package.mask 

# Restore binary version
emerge -uak foo
</code>