These Superview eyepieces are designed to work with scopes that produce a convex focal plane, refractors, SCT's and Maks. In Newts, concave focal plane, these eyepieces will show both field curvature and astigmatism because of the optical mismatch. The faster the Newt, the more significant the aberrations will appear. And as with all eyepieces, different focal lengths will perform differently. This EP design also performs better with a slower f/ratio refractor than a faster one. That's the design.
I have the 30mm and 15mm Superviews. The 30mm in my Newt is poorer in my Newts than the 15, showing a lot more astigmatism and field curvature. In my SCT, both EPs perform a whole lot better. But, despite the optical mismatch with Newts, for their price these represent excellent value for money and still do a good job. They are also very easy to use, especially for novices and as a first 2" eyepiece.
The 30mm was they first 2" eyepiece I bought for my Newt. Great to start out with, and my novice eyes didn't have any issues with it. I've bought and sold MANY 2" eyepieces since, but I never sold this 30mm, nor its 15mm little brother. I've kept these as they are excellent for outreach. Yes, these are not my first option EPs for my personal use anymore, but these have cemented their spot in my EP case.
A good mate of mine also bought his 15mm and 30mm Superviews at the same time as me, and uses them on his only scope, an f/4.5 Newt. To his good fortune he found his EP Nirvana with these, and has never felt motivated to change. Lucky devil!
. He well knows of their short comings, but he's most happy with them.
These are not a "perfect" eyepiece either optically in refractors either, and not for everyone either. Some people wouldn't look sideways at them, but other people wouldn't change them either. They have a place in my kit today, and when they had pride place when I first bought them they served me well. If $ are a concern, these are a very good option. If you are starting out in astro these are a good place to start, and you may find these are all you need.
Alex.