Yes, the synthetics are worthwhile. In terms of effects, at least. If it turns out that they're all terribly lethal, then of course their effects matter very little in weighing up their worth. I've sampled a range of these, but the only one I've had as a known quantity (i.e. bought as itself, not in a blend with secret ingredients) is JWH-073. I find the high from this has more of the euphoric (mental and bodily) aspects of cannabis, and some mild cannabis-like psychedelia at higher doses, but with less dissociation or confusion than cannabis. A very nice and satisfying cannabinoid, in my view; and one that betters cannabis in some respects (or, if you will, for some occasions: sometimes I do want dissociation, say to facilitate more full absorption in the more psychedelic aspects of the experience). So I would miss it, were only cannabis available.
None of the blends I've used (Spice/ Spice Gold/ Spice Diamond, Smoke, Genie, Tropical Synergy, Arctic, and a few others) have been quite like JWH-073; so I'm guessing if any of them have some in them, they have something else on top. I think I'd be sad not to have Smoke, which impressed me from the start, as giving a clean buzz much purer and less involving than cannabis (useful in some circumstances). Spice made me feel a bit droney and was physically uncomfortable at times, and Tropical knocks me out such that I have a hard time waking up the following morning, which is annoying. So I'd happily do without any of them.