I just got an FN L1A1 (7.62 NATO). I love it. I've got an AR15 and carried an M16 professionally in Infantry units....
The L1A1 with ammo is a heckuva hump! Heavy rifle, heavy ammo. As a did some hiking with it, I became much more fatigued with it than I ever did carring the AR15.
Ugh... for that kind of extra weight, I'd rather be carrying a GPMG like the M60 and have a serious increase in firepower for all the extra weight.
5.56mm has kinda re-earned my respect.