Salvia is nothing like weed and really shouldn't even be part of the conversation. Like someone already said... it's a psychedelic... it's much closer to lsd or psilocybin, but really it's not much like those either. Erowid classifies it as an "atypical psychedelic."
For a long time most legal herbs have been a joke. Recently, there have been a few that are very active... K2, K3, Spice, etc. However, the active ingredient in all of them is JWH-018 (or a similar JWH chem). The different herb mixtures are just inert plant material used as a carrier for the JWH-018. JW Huffman, the man who invented it, has said that he doesn't recommend smoking it, because of 1. how readily avaible weed is/how easy it is to obtain and 2. because we don't know all that much about the JWH chems yet, and they could be quite bad for us.
I smoke weed fairly regularly.... I've tried both K2 and K3, because a friend had them when he was on probation... Yes, it will most definitely get you high... and yes, it seems pretty similar to weed in SOME aspects... but at the same time, it's different, and it's pretty easy to smoke too much and get paranoid. There's not much of a body high or tired/lethargic feeling. It's more mental... but doesn't feel very clean. It can cause sweating/headaches/paranoia/etc., and it happens to people fairly often. I probably won't ever try it again. Not enough is known about how it affects us... Weed is more enjoyable and more safe from a health perspective.