Are you sure? I saw the revised response to the Sig brace a few weeks ago, but I have heard since then that they are now just a 100% no-go. Maybe this was just incorrect word-of-mouth; I need to do some checking...
Oh, and Kelvrick, that sucks.

I forgot than California doesn't follow the same rules as everyone else; in TN, stuff like SBR's and suppressors are no big deal, so long as you're prepared to wait a month or two and pay the 200 bucks.
edit: Okay, yeah, looks like Sig braces are still legal for sale; nobody's pulling them off the shelves or anything. Which makes some sense, since it would be like gun shops stopping the sale of SBR uppers (as in, 'SBR length; I know an upper itself does not constitute an NFA item).
Personally, though, I would not touch one with a ten foot pole. It would be like owning a smorgasboard of different AR parts...rifle lowers, pistol lowers, short uppers...yeah, yeah, as long as you don't assemble them in an illegal way, you should be fine. And even if you did, it's not like a silent alarm goes off to notify the ATF.
...but 'constructive intent' is a sonofabitch, and I
do not have any interest in opening myself up to even the remote possibility of being prosecuted for that shit. If I owned an AR pistol, it would be of the Olympic no-buffer-tube design, as shown above. For the same reason that if I owned a legit SBR, it would stay assembled and in a locked gun case at all times, just so there could be no questions about its parts being used with anything else. Call me a patsy to 'the man' if you want; I simply just still have an appreciation for the vast amount of firearms freedom I have, even if it does include some stupid rules that I want to make sure I never find myself on the wrong side of.
For these same reasons, I would never bother owning something like the foreign-made semi-auto SMG's (I know that's a silly way to word it, but stuff like the PPS-43 still says 'SMG' in my head) with a tack-welded folding stock. Essentially, if I'm shooting it at the range and the tack weld breaks, I could theoretically be put in fucking jail for it...fuck. that.