This one

Has anyone used this and could comment on how well it works?  I know, I know, get a dedicated lower.  There's a method to my madness here, I'm looking at registering an SBR.  If I do that on a standard lower, I can temporarily swap uppers for variety.  Plus I've already got a lower.

Worked great, had a drop in on a sbr lower, same as you

I have the bottom load with button head retaining cap screw from Macon Armory, which I believe is from the same production shop, in my SBR and it works great... I do shoot my dedicated Glock lower PCC for more matches..

JMO - I'm not saying this to be a dick or get anyone all butthurt or their panties in a bunch, but I didn't find the machining/finishing to be all that spectacular.  The few samples I saw were quite rough with lots of machining marks.  The two halves were also not quite machined the same so they did not fit together perfectly.  The first one I received was also not square and the thin sides were wavy - it would not fit in multiple lowers without significant force.  The replacement was better, but still a tight fit in several lowers.

With that said, once installed, the machining marks and poor finish are not visible and the block seems to work well at it's price point.  I have enough AR9's for now and if I decide to build another, I'll go with another Hahn blocks even though they are more expensive.  My comments are based on a KAK top loader that I received before they switched to selling the bottom loader that Rudy designed, but from what I've seen of other reviews/pictures, it looks like the quality of both varieties is about the same (hit or miss).

I've had mine (bottom load) for a couple of weeks. Works great so far.

PSA mags feed fine and drop free

C-products mags feed fine and don't drop free.

I've had one, bottom install, about a month now.
First 200 rounds ran 100%.
Aero gen2 lower with PSA complete upper, Metalform mags marked Brownells, Colt, PSA and Metalform.

The two halves of the mag block did have some mismatch and machine marks on the outside in areas that are hidden in use.
The fit into the lower and the fit of  the mags once installed  is like the block was made just for this set up.

So far very pleased, but about 19,800 more rounds are needed just to be sure.