Like most other guys here, my experience with Caverject in any form resulted in pain and aching, from the drug itself- not the injection. I use Trimix 30/1/2.5 and Bimix 30/1 alternately now. The Trimix with the Caverject level at 2.5 is ok, still very minor ache, but much better response to the med. Bimix doesn't ache, as such, just gives a slightly warmish/full feeling, no pain or discomfort. Whether taking any of the 3 drugs, it is a trial and error process with each, and each acts differently that the other and even from man to man, so telling you what levels to use or what your results will be won't work. We are all individuals and react as such. Bimix gives me just a smaller response as compared to Trimix. The levels that I have to use Bimix (1.0 ml) to get a response compared to .35ml with Trimix is a massive dose of either to most men here. The erection will last anywhere from an hour to 2 1/2 hours, and are usually easy to regain with manual or other stimulation, allowing for multi orgasms in those fortunate men capable of them, or at least an extended session for most. 
The injections themselves are virtually painless. Pain usually result swhen the injection is off and you hit other parts of the penile anatomy, such as going through the cavities into the middle membrane or too close to the head, etc. Using a 5/16 inch 31ga 1cc insulin syringe seems to be the best combination for most guys here. There is a fear factor of putting that needle up against your penis the first time or two that must be overcome, but when you are tired of impotence and want to rehab or have functioning erections again, its a small thing and easily overcome, especially after the first good one after so many months without... 
There's several techniques to injecting and hitting the right spot in the penis, depending on the size of the penis. Smaller and/or thinner ones take a different approach than larger ones. There's lots of info detailing that subject here, so some reading will be in order if you decide to go the injection route.