OP: I can only tell you that I've had my Sparc for 8 months, and it has maintained a zero through a couple thousand rounds of 5.56 while mounted on my Sig 556. I've also removed and re-installed it a couple of times during that period, and that has also not affected the zero. I'm still on the original battery. I've used it in bright, full sunlight, and the dot is fully visible.
Gripes (such as they are): the dot is slightly oblong - it's not a perfect circle. The 2x magnifier that comes with it makes this much more obvious than when it's unmagnified. I have considered getting a flip-in/out magnifier in 3x or 5x, but have held off because the non-circularity of the dot would probably get annoying when really magnified. But the non-circularity is hardly noticeable at 1x, and given that it is a 1-MOA dot, it allows much more accurate shooting sub-100 yards than I can deliver.
Also, as noted above, the mount is not a quick-detach rig. You do have to take an allen wrench with you if you want to pull it off. But I've found it to stay very firm when tightened, even without any loctite or other adhesive. The multiple shims made it easy to co-witness with my iron sights (mine don't flip), so that was nice.
All in all, I am extremely satisfied with the Sparc, and feel it is great value for the money. If I needed something to survive an IED blast, I'd shell out for the Aimpoint or Eotech. If I was mounting it on an airsoft gun and didn't worry about holding a zero through recoil, I would get an NcStar or something. But for recreational/weekend .223 shooting, I think it's right in the sweet spot.