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Hi Mike,
I hope you're well.
I have one of the early ones, which I got in 2007 or 2008. The newer SQM-L is obviously a slight improvement, but the old one still does the job and is pretty accurate. It certainly serves the purpose. I have found that the time of year, which affects milky way position has an effect on the readings. From the exact same dark sky location (Warrumbungles) I usually get readings of 21.4/21.5 in early autumn with, Crux, Carina, Centaurus etc overhead compared to 21.7/21.8 in October / November when the Milky Way is less prominent.
The important thing is to use the same device keeping changeable parameters as close to the same as possible, you can then compare relative readings and the absolute number itself is less important.
Cheers
John B
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