I'm curious if anyone installed and used one of these uppers yet.

Do a quick search for 'beowulf', there have been numerous posts, or visit the 'AR Variants' forum here on ARFcom... It seems quite a few of us have the Big B up and running.  My Beowulf is setup for whitetail and hogs and judging from my initial tests, it should hit like Thor's Hammer...

I love my 'wulf!  I think that it makes a lot of sense for a lot of citizen and law enforcement shooting situations.  
Rifle accuracy, shotgun sabot payload, AR ergonomics.
Downsides?  Some have expressed concern about the reoil.  I use an Vltor stock with rubber buttpad and a vertical foregrip and recoil is not a problem at all for me(or most others, apparently).
Magazine capacity is about 1/3 that of 5.56 when using AR mags.  Not good for the average infantryman, but plenty for the vast majority of citizen and le engagements.
Factory ammo is about $1.25/round, but reloading dies/components are available.  

So have you got one yet?

I have one with an A2 stock the stock cuts into my shoulder pretty badly, mostly because of the sharp edges of the stock.  I'm planning on getting one of the ACE kits with a rubber recoil pad or soften it.  Other than that its great.  VERY accurate.  At 50yds with my EoTech 552 on it I was making 1 ragged hole.  They claim it kicks like a 20 gauge shotgun.  It feels closer to a 12gauge slug gun to me (I have a Rem 870 w/20 slug barrel and it feels about the same as shooting 1oz and 1-1/8" slugs).  If you going to shoot it a lot reloading is the only economical way.  400gr factory loads are about $40 per 20 rounds and 325gr and 334gr are roughly $28 for 20 rounds.  If you want a more versatile large bore upper go with the .458 SOCOM which uses rifle projectiles instead of pistols.  A .50 Beowulf is really just a longer .50 AE.  It uses a 7.62x39mm bolt and a standard AR carrier with a modified .223 upper (no dust door) and the ejection port opened up.

Edited to add that you can now get brass from MidwayUSA.

I looked into getting a .50B, but went with a .458 SOCOM instead. There is a greater variety of bullets available for reloading, cases are easy to find, and it requires no magazine modifications to make it reliable. I have heard that mag mods are not always required with the .50, so YMMV. Another reason I went with the .458, is in todays political climate, and the hatred towards anything .50 caliber, it may take a bit longer for some dumbass politician to come up with a BS reason why they need to ban weapons like my .458. I have read where the.500 S&W Mag is in the hotseat now because it is alleged to be able to penetrate Kevlar vests, like the ones worn by LEO. Everyone knows that a .50 BMG will shoot down orbiting satelites, Airliners, and demolish an Armored Limousene.  In a nutshell, if it has .50 in the name, we must ban it!  
I think you get the point I am trying to make.