When I was travelling in Thailand a while ago I bought a pack of injectable diazepam (10mg/2ml).
I took one amp IM (I dont like IV), and i can tell you that the inj is really painful, but the pain subsides after a coupla mins. The solvent was probably Ethanol and/or propylene glycol.
The effects came after approx 1/2 hour, and felt just like swallowing a 10 mg tab, so the IM injection is just a waste if time and money.
A friend injected himself with an identical amp., but he took it IV, and he said that it kicked in in just a few minutes, but that the effects was just like oral diazepam.
The short acting benzo midazolam (Dormicum, Roche) is specifically made for injection, and I've heard some good reports of it. I have only tried the tabs (available in Asia), which is like rohypnol but lasts just a few hours. Great buzz !