I have the Long Perng one as well. It is great for eye-relief and is nice and sharp. I assume it, the Williams and the Orion "edge-on" planetary are just re-badged from the same factory. Mine looks identical with the Orion (selling at Bintel for $119)- but the Williams and the Long Perng have slightly different grips now.
I have since bought the Nagler 3-6 zoom and retired the Long Perng: it is on my desk staring accusingly at me as I type this...
I am very happy with the Nagler: unlike the comment from JohnB I have noticed no off-axis drop off, and the 3-6 zoom is wonderful. It does have shorter eye-relief though.
If you want, I am happy to sell you the Long Perng
: say $40 + postage (it was $99 when I bought it, but Andrews have dropped their price). Drop me a pm if you are interested.
All the best,
Dean