A while ago, I promised someone (but I've forgotten who and where), that I would check out, whether a Paracorr improved cheap eyepiece performance in a f/5 newtonian.
I had a chance to test it last night. I used a 12" f/5 Meade Lightbrigde, a Televue ParaCorr and a 20mm GSO Superview erfle eyepiece. First I checked with the eyepiece alone. As expected, the edges had the typical "seagulls" from the combined coma, field curvature and astigmatism. In the center 50° it wasn't bad at all, and the inner 40° was very sharp, but the edges got exponentially worse after that. I inserted the Paracorr and took another look. Now the stars along the edge were better, but they still weren't sharp at all, due to the field curvature inherent in the eyepiece. If I focused the stars near the edge, they turned into short streaks instead, due to the off-axis astigmatism because the erfle can't handle an f/5 light cone off axis. When the center was in focus, stars around the edge were sizeable blobs. Smaller than the "seagulls", but nowhere near a sharp point.
The overall impression is that it wasn't really worth it. If you're using low-end eyepieces on your f/5 or faster newtonian, don't expect a Paracorr or other coma corrector to save them, because it won't.
I also compared with an 18mm ES82 and the difference was ridiculously large. The performance of the 18mm ES82 *without Paracorr* was vastly superior to the performance of the 20mm GSO *with* Paracorr...
Bottom line: Get a line of good flat-field eyepieces first, THEN a coma corrector.