I've got several "alternative" calibers in AR-15 uppers. I have a DTech upper in 25 WSSM. Mike definately builds a great shooting rifle. I've got an Olympic Arms upper in 243 WSSM, and although heavy, it's a good shooter. My wife has a 6.5 Grendel and it, too is a fine rifle.
I looked at the AR-10 platform and was seriously considering it, due to the variety of calibers that can be used in it. I was able to pick one up and handle it alongside the Oly 243 WSSM. Absolutely no comparison in size, weight, and handling. AR-10's are big. If I wasn't going to pack it all day long, day after day while chasing antelope and coyotes, I'd go with the AR-10. However, I like to cover ground on foot. That's why I went with the AR-15 platform.
Mike Milli at DTech has some new calibers now that are really impressive. My next one will be either the 6DTI or his 6.5DTI. In fact I sent the money off today for a new scope...for the rifle I haven't ordered yet! They are based on the 6.8 SPC case, necked down with the shoulder blown out.
Olympic Arms came out last year with a slightly longer lower and chambered their AR-15 in 22-250. The magazines are longer and this will allow us "alternative caliber" shooters to seat those heavier bullets out where they belong and get a bit more powder behind them. I'm pretty excited about it actually. It will really open up more bullet options, and of course all the wildcats that can be built off a 22-250!
Oh yeah, just to clarify, the magazines for the WSSM's and the 223 are all the same. Just the followers are different. I've converted several for my WSSM's.