I like the concept of the 6.5G also.
I jumped in whole hog and had an Overwatch upper and built an 18" SPR also. Getting components was quite a pain but I finally get it done.
My biggest issue came to the ammo itself. Commercial ammo was either out-of-this-world expensive or crap (I found the Wolf Gold to be crap, in my experience...YMMV). I found the cartridge to be very finicky in my reloading experience...I could find a load that would be very accurate but not optimal in other respects (e.g., velocity). Components were also very expensive with few sources carrying them.
In the end, the cartridge is a great idea...but you have to be pretty dedicated to be a user. I thought I liked odd-ball things and am usually willing to work outside of the mainstream. I wasn't, however, dedicated enough to be a long term 6.5G guy. Nothing wrong with the products...just a bit too much work for me.
I switched to the 6.8 SPC and am very happy.