Tried to run 1.2.1, but it stalled on the last step (installing the nightly build) with an applescript error "Volumes://.app not found."
Located a copy of 1.2.0 on the dev's website, and it seems to run fine. I'll stick with that till the problem's resolved. I'm using latest everything (OS, equipment, etc).
As for the product itself, I can't really tell that much difference from the stock Safari but the pages do seem to load a bit speedier, which is nice. For those who don't mind being on the bleeding edge (and taking a chance that a bad nightly could cause browsing issues), this is really quite handy.