I've been absent for some time when did these become readily available? Let us know how it goes
As for it replacing the M4/M16 as fishbed said it's a long long way away from that. It's not just a question of better weapon system here but does the Gov't really want to spend all that time and money required to replace the existing weaponry for something that does the same thing.
I've never fired a bullpup and always thought prone would be much more difficult with a 30rd mag positioned at the butt stock. I have pulled the trigger on an FS2000 and they still have a long way to go.
Overall i've just grown very fond and familiar with my DI ARs.