I have the Vixen 8-24 zoom and I really like the convenience of this eyepiece. The images look quite respectable to me at f/10 and f/6.3 but I don't know how it would perform at f/4. I use a 32mm plossl to get a wider low power field as the zoom is only 40 degrees at 24mm, 60 degrees at 8mm. The only other eyepiece I use regularly is a 17mm Vixen LVW.
If you search the net you'll find reviews done by qualified reviewers and you can find user reviews at
http://www.excelsis.com/1.0/catalog.php?categoryid=3
Hope that helps,
Tony