It's a lot easier to change them with the intake manifold off. It's not tough to do... Couple of vacuum lines, and a dozen extra screws and you've got a clear shot at 'em.
But BE CAREFUL!!! The brass inserts molded into the housing don't like to be twisted very hard, and the housing is not available seperately. The housing assembly (including both sensors and the thermostat, is $250+.
-Joe