Yes, if we look at Duet Display's reviews on Apple's App Store, it does seem that most customers are pleased with their purchase and find that it was a worthwhile investment in increasing their productivity.
It is ranked #32 among Productivity apps, and out of the app's 31.3K customer ratings it averages out at 4.4 out of 5 stars.
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Duet Display
Use your iPad as a second display for your Mac