The ARRL 10 GHz and Up Cumulative Contest is the only fully microwave spectrum (above 1,000 MHz) communications contest sponsored by the ARRL. It chiefly finds participants in the Northeast, Southern California, Florida and the Midwest. This is due to the geography and operator density of these areas. There is also quite a bit of club competition in this contest which is helping to keep the contest alive and exciting.
Tony, KC6QHP, has taken the time and written a really, really good write up about what Southern California operating is like, including operating from a former Nike missile site! Give it a read and get jealous!