I've intensively tried three different sets of eyepieces for binoviewing, and they are:
1. Japanese Widescan Type III 20mm: they have decent eye-relief, very comfortable without glasses, no kidney bean distortion so that they are very easy to view through. They have more than 80 degrees field of view and one can see to the field edge without problem. They render good image but the field edge could become soft, and the light throughput can be better.
2. Japanese Orthoscopics 18mm: they have good eye-relief too, and very easy to view through but since they didn't get an eye cup, ambient light will affect your observation and it also makes them a little bit hard to hold the view. These eyepieces are extremely sharp, really excellent for planets, moon and the sun. But the field of view could be a little bit limited for deep sky objects.
3. Tele Vue Plossl 20mm: they are very comfortable to look through, they're extremely sharp and the eye cup is very well designed, their field of view is wide enough not to be frustrating, of course, I guess 60 degrees+ should be even better.
Over the years, the 18mm Orthoscopic pair is used least, mostly due to the absence of good eye cups. The Tele Vue Plossl pair used the most, since it's sharper and more contrasty on planets, the moon and the sun, and they have better light throughput for deep sky objects, too.