actually, there isn't a large difference in the amount of time it takes the drug (any drug) to hit you. comparing IV and IM.
IV ketamine will hit in about 10-20 seconds, you will taste it in your mouth almost as soon as you push the plunger.
IM ketamine will hit in about 40-60 seconds, but you won't ussauly taste it.
IV opiates(nubain) will hit in about 20 seconds also, and IM only takes about 30seconds.
IV opiates hit you a lot harder (i.e. all at once) lasting about 20-30mins and decreasing after that.
IM opiates are a much more steady release, lasts longer.(1-2hrs)
only "bad" thing about IM is you run the risk of an abcess (infection). but you can also get infections through IV use also, but ussauly toxins exit through the lungs.
BUT REMEMBER, MOST WHO GO IV FOR THE 1ST TIME, NEVER GO BACK TO IM. (like me, sadly, but i'm slowly beat'n it).
~hope this helps.