I wanted a bullpup shotgun, the Keltec is unobtainium, so I researched the Bullpup Unlimited (BU). Got an ok deal on an 870 tactical and ordered the bullpup. I cannot remember where I read it but I had heard that there were some betterments made to the fit, finish, and polymer of the BU. One of the popular distributors had the BU but I ended up purchasing directly from BU directly to ensure I got the latest model. All told I spend about $750 or so.
Assembly was relatively easy, directions are put together pretty good. I found that it was easier to get it all together if I left a little play in the screws during assembly. I put it all together then tightened everything down.
I really like it. Short, compact, fully loaded it's a little bulky, but the ergonomics are good and it shoulders very easily. I did a quick function test and it ran well. Recoil was tamed a lot by the squishy buttpad. It was a lot less than I expected and far less felt recoil than shooting a stock gun.
One thing that I did notice is that the manual of arms is different. The slide release is a button towards the rear and I haven't quite figured out how to manioulate the weapon.
The good thing is no permanent modifications to the shotgun. I've got pics, just nothing available right now.