Seen some good videos on these. Do they really work that well with all mags?  From the reviews, it sure seems so.  Are they perhaps finicky with what upper brands you use?

Gonna Form 1 my favorite AR pistol and was thinking of building upper in 9mm and buying one of these.  I see mag release is in a unusual spot but son and I are both Lefty's so it's all good. Lack of last round bolt hold open doesn't bother me.  It'll be a range and pinking toy

Got a Fostech binary trigger in AR lower. Will it cycle 9mm?

With respect to the Sylvan Arms (SA) Glock magwell adapter for the AR, I've had one for a few years.  I bought it specifically because it does not have LRBHO.  I don't want to mess with installing a spacer in the buffer tube to shoot 9mm, and removing it when I shoot 5.56.  When using the SA adapter, I stretch an o-ring over the standard mag release button to prevent inadvertently dropping the adapter out.

The SA adapter I have works well with KCI and ETS mags, but factory Glock mags are tight; of course factory Glock mags are somewhat tight in my Glock pistol.  I have sanded down all my Glock mags so they at least drop free from the Glock pistol.

MHO, YMMV, etc.

Thanks for the reply. I'm leaning towards getting one. They say this is a Gen 3 model and I'm seeing them for $90 which seems like a good deal. So, it feeds and functions reliably?

At the time I bought the one I have, Torx and Stern were the only other Glock magwell adapters - ATI hadn't released their version yet - so I paid the same as what the Stern adapters were selling for at the time.  I have no idea what "Gen 3" is, how their Gens differ, or what Gen the adapter I have is.
As for feeding, that depends on several factors including, but not necessarily limited to: ones barrel chamber entry configuration, the geometry of the magwell (for dedicated lowers) or the adapter geometry, the magazine, and the ammo geometry.  As well as the SA adapter, I have a dedicated Glock mag AR lower. And I have two 9mm AR uppers. No combination of uppers and lowers feeds flat point ammo very well.  As I only shoot 9mm round nose ("ball ammo") in the AR pattern rifles, I've no problem with leaving hollow point and flat nose defense type ammo to a Glock pistol only application.
Best of luck whatever you pursue.
MHO, YMMV, etc.  Be powerful; be well.