Tablet had a short feature on Z Street, which came about as a right-wing response to J Street, which was a left-wing response to AIPAC, which somehow convinced Americans that it was the only group qualified to talk about Israel...
Personally, I don't much care about any of this alphabet soup. The little I've heard about J Street seems to indicate that yes, it is fairly left, and seemed to make some mis-steps during the last war in Israel. However I would be more inclined to attribute that to some boneheaded errors in judgment as a result of being a new organization that isn't yet used to playing rhetorical politics as opposed to being indicative of an Israel-hating agenda.
As for Z Street? I know even less about them. Their positions mostly seem to boil down to wanting to correct misinformation (a worthy goal, if done right), advocating the right of Jews to live anywhere (pro-settlements?), pointing out double-standards applied to Israel (fair enough), and, perhaps most interestingly, "Categorical rejection of agreements with, or concessions to, terrorists (or their supporters) who are dedicated to Israel's destruction."
On the one hand this makes intellectual sense; on a practical level I'm not sure where it leaves us as far as dialogue or peace-making. Then again these days that seems to be a fairly theoretical issue anyway.
When it comes down to it, I have continually been surprised by the fact that, as vocal as Jews as a community are about countless issues, including the Middle East (remember this joke?), that on Israel, a single organization has claimed for decades to be truly representative of the community's viewpoint. As far as I'm concerned, there should be more voices in this discussion, if only to show that there are common grounds the Jews do agree on, and also perhaps to moderate some of the different parties (though, of course, it is always possible the exact opposite could occur). J Street, Z Street, upside-down polka-dot street, whatever. The more Jews involved in Israel/peace activism, the better the conversation will get-- both internally, and externally.