I have been a frequent pot smoker since college days, 40+ years ago. I love the stuff - it's my medicine. But during droughts I've just had to go without weed for months at a time, and I welcome the availability of the c-noids.
FWIW, here are my experiences with a few synthetics, 073 & 018. First, personally I find the c-noids come on slowly compared to pot. One of the things I've always liked about weed is that I can feel it take effect just a few seconds after my hit - my leg muscles relax first. With this instantaneous effect you almost never smoke too much at once...if you're stoned, you simply decline more. A wonderful self-regulating effect. But for me (YMMV) the r.c.s come on slowly, like alchohol, so overdosing can happen if you take repeated hits before the first one shows effect.
I fight this by taking one hit and not re-upping for many hours.
I eyeball my doses. I taped a grain of rice to the side of the plastic baggie and I dip the point of a pocketknife into the powder and pull out a bit. I compare the powder on the knifetip to the grain of rice and start smoking with a volume of powder equal to 1/4 grain of rice. Measuring this way will give me about 50 hits from a gram, about a dollar a hit.
Tolerance for 018 does increase quickly, so I'd suggest hitting it not too often. My first gram lasted 25 days. The most recent one only 4. Perhaps it's wise to rotate one's experiments with drugs so that no single substance becomes central.
Personally 018 was significantly stronger than 073 for me. I kept a log of effects of the 018 and onset began about 5 mins after the hit. The first effect I've noticed is a ringing in the ears, which soon dissappears. Thereafter the stone sets in. Good for 1.5-2.5 hrs. Pleasant but a touch "thin", lacks the full sensuality of good pot.