The odd thing to me is that if the BiMix, or Trimix are so much superior to Caverject - why do the Urologists always prescribe the Caverject.
The cynic in me says they make more money from Caverject. Another possibility that you noted earlier is that they just don't know about Bimix and Trimix.
Try asking your doc for the prescription. The Bimix mixture can vary between 30/1 and 30/5 (papaverine and phentolamine) -- depending on who does the prescribing.
I'm on a 30/3 mixture, and for me each injection is a fairly low 0.2ml.
Priapism (rock solid erections lasting more that four hours) is supposedly more of a problem with Bimix, but has never happened to me. For this reason, I suggest start with a low dose and work your way upwards.