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As is the case with all eyepieces, performance varies across the ranges.
In 100°, the 5.5, 9, and 14 are quite good, while the 20mm suffers from a lot of edge astigmatism and the 25 has a lot of problems, including vignetting.
In the 82° series, edge of field astigmatism is the primary problem, but it is less in shorter focal lengths. The 82° LER series has a lot of scattered light and chromatic aberration problems.
The 68° series is very nice in 16, 20, and 24mm, but suffers from significant light scatter in longer focal lengths as well as edge astigmatism.
And, it's the same in the 52° and 62° series as well--best to read reviews of a particular focal length because they vary in performance across the focal lengths.
So, overall, ES eyepieces represent good values, but it depends a lot on the f/ratio of the scope (they perform a lot better at f/8+) and scope type (people notice field curvature in refractors, often), and focal length of the eyepiece.
I do have to add that, by and large, they are significant upgrades over the entry-level eyepieces, like the ubiquitous 66°-70° eyepieces that can be bought for very low prices.
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