ok i'm gonna list what i've plugged and my results, because you all care so much.
oxycodone - better than sniffing, but you build a tolerance quicker this MoA than sniffing/swallowing IME, IV'ing builds a tolerance the quickest of the four MoA's IME (and obviously it would just out of the math of the BA's)
focalin (dex-methylphenidate) - better than sniffing/swallowing, a nice rush, relaxing and calming but also clears the mind, very similar to IV'ing a low dose of coke or a decent dose of ritalin/focalin.
cocaine - you miss the enjoyment of sniffing a nice fat line of coke and the smell and that whole process but plugging C provides a very nice IV-like rush and calm waves of relaxation and euphoria all throughout your body if you plug the right dose of decent cocaine. i find that when i get a G of coke lately i'll end up plugging 1/3 to 1/2 the bag because its that enjoyable. and also i'm trying my hardest to avoid going back to the needle especially because of the drug i've been using daily (listed next).
heroin - since i'm not sure what quality or type of dope i'm getting (i'm relatively new to the H scene, used to pharmaceutical painkillers). but if i use 1/4-1/2 a bag of dope plugged i feel the shit kick in around the 4-5 minute mark and it feels very relaxing and euphoric and is quite pleasant but i'm not sure its as effecient as sniffing. smoking (or vaporizing since i'm using foil and chasing the dragon) seems to work with the shit i've been getting which is why i say i'm not sure what type of dope it is, #3 or #4 (from NYC, so i'm assuming its #4 but it smokes quite well and gives you an insane rush after it liquifies and then vaporizes away. anyway plugging H so far has been not as good as all the other drugs i've tried plugged so i have been sniffing my dope lately, no more smoking or plugging and no IV'ing!
valium/xanax - not worth the process IME/IMO. although it does give a slightly more intense rush with a quicker onset the BA is only 3-5% higher than oral so i stick to eating my valiums (and the occasional xanax bar i get).
diphenhydramine - only tried it once, didn't think much of it, so i stick with oral (or IV if i'm shooting but i'm not and i'm not going to)
morphine/codeine - i have been plugging my opium tincture on a semi-regular basis. it's 10mg of morphine / mL and i get 3mL a day (30mg morphine). i find if i plug 20-40mg of morphine it's on par with sniffing half a bag of the dope i've been getting. and equal to drinking about 60-80mg of tincture (plugging is 2-3x as effecient as taking it orally IME)
amphetamine - although i haven't tried my friend and drug experimenting partner says its similar to plugging ritalin/focalin or shooting amps/coke but obviously not anywhere as good, but feels somewhat like IV'ing. i've yet to try because i hate amphetamine , makes me too jittery and anxious.
i'm sure i've plugged a few others but i can't think of them atm... peace.