JWH-018 and related compounds JWH-073 and JWH-081 are synthetic cannabinoids that are currently completely legal in the US. They have effects very similar to THC (marijuana) but (for JWH-018) about four times as strong for the same dose. They're commonly sold sprinkled or melted onto non-psychoactive plant material so they can be smoked like marijuana, though most brands emphatically state that the substance is "FOR INCENSE USE ONLY, NOT FOR HUMAN CONSUMPTION"
The most common brand of JWH-sprinkled incense is Spice; it's sold in gas stations and head shops as a "herbal incense" for varying prices, usually between $10 and $40 a gram. Online, pure JWH compounds can be found for prices from $40 to $50 a gram - but the pure compound's usual dose is only 4mg, so one gram is 250 doses. Pure compound is not usually smoked, it is taken orally.
Few studies have been done on the JWH-series synthetic cannabinoids, but it is known that their activity in the brain is almost identical to THC, but with greater affinity to the CB1 receptors in the brain. It is difficult to say what long-term effects of JWH-series use are, as the chemicals have only been popularly used for a few years; however, they seem to be identical to the effects of cannabis usage.
What this means plainly is that JWH-018 is, basically, legal (and more potent) marijuana. Its easy and cheap availability online also means that doses of JWH-018 are much cheaper than doses of marijuana. Note that I'm not advocating its use here: it is, after all, an untested research chemical, and should be used with extreme caution. While the long-term effects appear to be harmless, it really hasn't been around long enough for this to be known for sure. All cautions that apply to marijuana use apply double here.
Personal notes:
I smoked a bowlful of Spice 50x in a (tobacco) pipe, around 9:30 at night. I started noticing effects about 5 minutes after finishing the pipe. The first sensation was lightheadedness and a feeling of warmth spreading from my head down. It went into a feeling of joy and creative potential, as well as considerable appetite increase (munchies,) much like I've heard pot's effects described (I can't really give an accurate comparison; I've never tried pot.) Moving around was strange - there was an almost slow-motion feel to the world, and I had to concentrate to focus my eyes properly. I listened to some music (Guns'n'Roses' "The Garden") and my enjoyment of the music went waaay up from normal. The high lasted a couple of hours, but how long exactly I'm not certain - I fell asleep about an hour and a half from smoking. I noticed that I fell asleep much easier than normal (I'm a chronic insomniac) and dreams were enhanced a lot.