I purchased a Kushnapup stock for my Saiga 12. These are my impressions.
Installation: Installation was straight forward, though instructions that include which screw goes where would be nice. Some people have cut a slice off of the original buttstock to act as a spacer so that they don't have to saw off the tang. I've built AK's from kits and have no fear of removing the rear trunnion and installing another if I decide the bullpup stock isn't for me. I cut off the tang. Total installation including adjusting the trigger- about half an hour.
Balance is OK. With a loaded 8 round mag it's really butt heavy, but the short length is handy and it sits well in my hands and shoulders very nicely.
Learning the controls around the new configuration takes a bit of practice, The safety is no problem, but the mag release is a little awkward. The BHO is VERY awkward.
First, the Saiga must now be removed from the stock for cleaning. One of the AK design's strong points is the quick easy field strip. Now gone, bye-bye. You have to remove all of the screws from the stock and then remove the barrelled action from the stock in order to remove the bolt. The dust cover and mainspring will come off, but there isn't enough clearance for the bolt. Cleaning is now a chore.
Second. The stock does not come with any type of optics mount. In the bullpup configuration the factor sights are unusable. So your going to spend over $250 for a new stock, get it in, install the weapon... and boom. No usable sights until you order a sight mount. Suckage.
Addendum: I ordered the CSS long rail that replaces the rear sight. NOT happy with this item. First, installing it is a chore. After I cut the old sight to relieve the pressure and got it out of the dovetail, the replacement dovetail wedge wouldn't even BEGIN to fit. I expected to have to file a bit to get it just right, but this was so far off it took several minutes to even get it close enough to start into the barrel ring. Second, once I got the base installed I discovered that the top of the base isn't square, so the rail doesn't sit on it neatly. The rail itself, listed on CSS's website as 4" in length, is barely 3.25" and wasn't straight. the rear couple of slots are definitely tilted upward. Not the slots I'm using, but the upshot is that I'm not happy with the fit, the quality or the fact that it's shorter than described by 3/4". At $50 it's half the cost of the Ultimak, but it's not half the mount. My advise is to spend the extra $50.