Spice is really only an alternative for the desperate or naive. It tastes horrible, has a shitty consistency that makes it difficult to roll with or pack a bowl, and has an uninteresting, two dimensional high. Add to that the fact that it's no cheaper than the real thing, and that its active principle is a novel and possibly dangerous chemical, and spice is a bum choice. The pleasure of smoking cannabis is not just in the effects, and there is no way that this sickly stuff can compare to the enjoyment gained from a delicious piece of high grade hash, or a well grown sativa.
As for the legality issue, JWH-018 is indeed of a completely different structure to THC; however, your analogue law covers anything that is substantially similar in structure or effect to a scheduled substance. Believe me, this vague catch-all law could certainly be interpreted as covering the synthetic cannabinoids in Spice (as well as pretty much any chemical under the sun)