Yes, they're perfectly usable on ARs, and will add some decent velocity to the 223s. Doesn't take a larger cartridge to "use" a longer barrel. we're talking about changing expansion ratios here, and increasing barrel length will benefit almost all centerfire bottlenecked cases. I use a 26" on one of my Match rifles (a Spacegun, as rhino suggested), and it adds a noticable velocity advantage over my 20" Service Rifles. You can get into issues on an AR about how well the receiver supports such a long barrel if it becomes too heavy, but ballistically, it's an advantage all the way across the board.
The flip side is true as well. The M4s, with their 14.5" (and even shorter) barrels are lighter, easier to handle and faster to employ than their 20" M16 counterparts, but they lose out ballistically. Less velocity, less energy, reduced terminal performance on the receiving end. Again, it's all just a matter expansion ratios vs. handling characteristics.