Sgt, It's funny that you mentioned 21st Century as their 1:6 scale offerings were some of the first collecting I did as an adult, the first time I ever collected 12" action figures. It was them, a few of the early 2000's GI Joe 12" figs, some World Peacekeepers and even a couple of the pricier BBi 12" figs, if I could find a "deal" on them. I even had (have) the Bradley, a couple AH-6s, and a few other vehicles that 21st and GI Joe offered back then. I really liked the authentic look most of those companies were going for with many of the figures/vehicles and pretty much avoided the more cartoony offerings.
But, like you mentioned, they were too big and took up an entire (small) bedroom office. And, when our firstborn came along and that became a nursery, they went into storage, never to be revisited until I finally let my kids get them out, recently, now that they won't destroy them entirely. It's amazing they held up as well as they have being stored in minimally conditioned attic storage all these years!
It does say something that I held onto them, never selling any until recently and never disposing of them. I appreciate them for there attention to detail (most of them at least) and their attempts at replicating the real deal, but I definitely see the limitations of the real cloth uniforms and the hard to scale kits and such. Plus, they are all pretty dated now, as camo patterns, gear and even some of the weapons have moved on.
When we moved to central VA and a larger house, and I got a home office again, it was right around that glorious time when BBi was putting out some pretty excellent 1:18 scale vehicles and aircraft. So I got all the ones I liked from those collections (quite a few on clearance at Tarjay!). Along with the 21st Super Cobra and the Spotcat I won (major award!) on here, they are my hanging collection in my (permanent, for now at least
) office in our current house. They really are the right scale for decent detail, without making a room completely unusable for anything else, and typically get some wows when people see them.