I've been smoking this one kind every night for about a year now. The previous chemical got banned, it was much harsher. Me and my other friends that are using it (and have been using longer than I) keep in touch about it and we haven't noticed any side effects or withdrawl symptoms yet. We're being the guinea pigs. From what we've experienced it seems to be up to how the users body handles it.
This kind called LOL is what i've been smoking.
Before that it was in this order:
K2 - (had a mini siezure, threw up, wierd anxiety/dreamlike high)
Red rhino - (Dreamlike high, noticable heart racing, kinda feel like you're on cocaine)
Mr. Smiley - (taste nastier than shit, smoother kind of high, threw up, mostly because of taste)
Stoopid - (Holy fucking shit. Stuff makes you Stoopid. Almost hallucinogenic. Threw up multiple times. Usually after a cigarette kicked the high up. Wasn't so much the taste as it was "Holy shit I'm high...blaaaaaahh") I still like this stuff in small quantities.
All of the above seem to be harsher than LOL so far.
Now Bath salts (think fake cocaine) are hell. That shit is really bad for your body. You don't feel healthy while doing it. I did a lot of it, stopped, and am still fine. My friend over did a bunch of it (He's one of those guys) and now has an irregular heart beat and anxiety issues. I know someone else whos friend over did it and died. The point here being don't over do shit, especially when it isn't that well known.