Q:

Does a luxmeter provide an accurate measurement of usable light?

Does a luxmeter provide an accurate measurement of usable light?
What values constitute equivalents of 'shade', 'part sun/shade', 'full sun', etc?

A:

Yes, low end (less than $100 US) lux meters will work for most measurements, and is a useful tool. 

Deep shade (too low for the average plant to thrive) is about 1,000 lux.
Normal house lighting is about 60-100 lux.
Full daylight (not full sun) (good for shade loving plants) is about 10,000–25,000 lux.
Direct sunlight (ideal for most plants) is about 32,000–100,000 lux.

