I have not tried spice, but I'm a master of weed, coonessoor of both Sativa and Indica strains, and I have read some shit about synthetic cannabinoids so I will leave my word here.
IMO, 90-100% Sativa excellent-potency marijuana is quite similar to spice. In my experience at least, a good dose makes things seem very shamanic in a sense, the environment takes on an almost divine importance. Things are not funny or silly, they are serious and grand. To me, it's like a minor psychedelic trip of its own kind but it can turn into a severe dissociative panic attack.
Too much of this shit, especially if you're starting out, will fry your mind a little.
I've had month-long after-effects from natural pot.
Especially as my tolerance has reached a very high point, I find this to be more and more true: if you smoke three bowls within five minutes, your peak effecs will be a lot stronger than if you spread the bowls out over 15-20 minutes (sometimes I wanna get my stone on but I'm too fucked up to speed-smoke properly).
So already we see differences, weaker sativa (>80% ) often has some CBD in it as well, but on top of that it takes some time to smoke. Good, potent, pure sativa can be smoked fairly quickly and impressive or frightening effects will be achieved. It can trip you out almost as much as real hallucinogens do, but tripping and being tripped out are two very different things.
Synthetic cannabinoids are very potent, so the risk of accidental overdose (problems, not death) is far higher. And most of the people who smoke it probably do it very quickly (an entire dose in one or two hits).
Just my few cents.
