Eyepiece testing is relatively easy on the PST.  One simply looks at the faintest prominence detectable on the solar limb in a given eyepiece (i.e., light throughput), judges contrast on surface features, and looks at the ability or inability of the image to "snap" into sharp focus.  The "prominence test" is particularly easy for a single observer to do with reasonable objectiveness between different eyepieces-- i.e., do I see the prom or not!

With the aid of another NOVAC (http://www.novac.com) member, in November 2005 we tested a number of eyepieces head to head in two PST's.  The contenders included:

While a 5mm is the 'theoretical' highest power eyepiece for the PST (going with the "50 power per inch of aperture" rule), I consider the 6mm a realistic magnification ceiling for the PST.

Neither one of use was happy with the results of the Cemax Barlow when used with the Cemax 12mm. We basically could not get a crisp focus. While the 7mm Type 6 Nagler showed great detail, its seven optical elements dimmed prominences so much as to lose the ability to discern the fainter proms.

Of the eyepieces we tested, the Nagler 3-6mm zoom, the TMB Super Monocentric 6mm and Nikon 9-21mm were the best. All three provided bright crisp viewing with excellent contrast.  Testing by Markus Ludes of APM (http://groups.google.com/group/sci.astro.amateur/msg/8ac5e1265b40bf86?q=ludes+transmission+eyepiece&hl=en&lr=&ie=UTF-8&rnum=2) reveals that all three have particularly high light transmission values.  The Cemax 12mm also did extremely well and was comfortable to use due to its 18mm eye relief. It appeared to be comparable to the Nikon zoom in performance, but the more comfortable eye relief would make it a winner for eyeglass wearers. 

Zooms rule!

I use a pair of the Nikon 9-21mm zooms in my binoviewer, which has a Barlow of 1.3X.  I have heard anecdotally that the zooms currently available from Apogee Inc. (http://www.apogeeinc.com/) are OEM versions of the Nikon.  The consensus on the web is that virtually any zoom-- Vixen, Televue, Orion, cheapie Chinese job-- renders a great solar view.  I suspect even those zooms not noted for strong nighttime performance do comparatively better with the extra light available in solar viewing, especially when paired with the convenience of zooming to match the available and rapidly changing seeing conditions.

I also tested one of the high end and physically massive (Nagler 20-sized) Pentax XL series 8-24mm zooms.  It equaled or slightly exceeded the performance of the Nikons. (See images and comparison at the bottom of the Eyepieces page)

Besides the eyepiece rankings and scope comparison, I would offer the following tips and techniques:

1) Use a bellows-style eyecup. When I move one between two eyepieces, it GREATLY improves the apparent contrast between the sun and the surrounding sky, and brings out more detail on the disc of the sun. (Orion Telescope and Edmund Optical make these; Orion's are much less expensive)
2) At least for solar observing (and consider it for those very faint fuzzies as well) use eyepieces with no more than five elements. I'm not sure why the five element Nagler zoom was able to keep up with the one element TMB monocentric in performance, since "less should be more" in solar as well as planetary observing. (Both the Nagler and Nikon zooms have 5 elements.)
3) Do not use the Cemax Barlow.

(Portions of this page have appeared on the Solar "Special Interest Group" of http://www.novac.com