I have a Ruger GSR Scout in the 18" Stainless version and like it, it handles and balances well. I didn't buy it for the scout design, I bought an XS rail (which preserves the back up irons) and traditionally mounted a scope, but it had all the other features I wanted, shorter barrel, stainless, 18", box mag, threaded, and back up irons.
In regards to the .450 bushmaster, I don't see the point unless one is restricted in hunting only with straight wall cartridges, which is what I think the market Ruger was going after. It doesn't really do anything that the .308 does and at a much cheaper cost of both factory and components. Looking at that article that was posted a few posts ahead of mine, the energies look similar and it lacks the penetration that the .308 delivered (at least on water jugs, which isn't a very precise indicator of real world but it says something).
But if you are in an area that has restrictions on rifle cartridges, then the .450 bushmaster in the Ruger GSR would be a great option. And I get it's value in an AR15 platform as a 'thumper' round, although I would lean more towards the .458 socom, but I digress.
Those are just my opinions, with all these new cartridges coming out one has to wonder in 10-15 years what will still be around. I know the .308 will be, the .450 bushmaster...I don't know. Like many of the new wildcats the AR craze has brought everything out of the woodwork.