There seems to be a problem now with installing STS2.3. It complains:
Missing requirement: SpringSource Tool Suite Package 188.8.131.52912171331-RELEASE (com.springsource.sts.package.feature.group 184.108.40.206912171331-RELEASE) requires 'org.maven.ide.eclipse.book.feature.feature.group 0.0.0' but it could not be found
It seems that Sonatype has made a significant new release 0.10.0. They say you must uninstall previous versions to upgrade.