This topic sure seems to be a moving target. Too many definitions;
Compared to more (or much more) expensive alternatives.
Obtained for a uncommonly low price.
Little known gem.
Personally I've found that a manufacturer's slower lenses offer good value. In Olympus OM gear I have built up a modest collection of Zuiko's; 24 f2.8, 35 f2.8, 50 f1.8, 85 f2 (no slower option here and my most expensive lens), 135 f3.5, and 200 f5 all shopped carefully for a total of well less than $500 over the years.
Other advantages are light weight and a common 49mm filter size.