Definitely worth it.
It is a good aero mod (hard to measure its
EXACT aero affect for most people, but it is better aero).
But of the
same, or greater value, is how it keeps the engine warmer, lets your auto transmission warm up sooner, and lets you turn your heater on quicker without impacting economy. This helps your Torque Converter lockup sooner in automatics (assuming Lumina is auto), and gets the engine running in what is called 'closed loop' for emissions.
All positive things in terms of fuel economy, both from what I read here, before I put mine on, and from my experience now, running with a partial one the last 2 months.
One thing to remember:
Radiators and cooling systems are over sized - designed for
worse case: towing up Baker Grade in California / Nevada / Death Valley
Baker Grade Google Maps
Las Vegas Road Trip - Baker, CA | out of las vegas
Baker, California - Wikipedia, the free encyclopedia
So, it is OK to close
SOME of the air flow off to the radiator, even if you do not have a scangauge etc. I don't have one yet either.