I used a 24-105L for years before upgrading to the 24-70 II...
It really is stunning how much more resolution the new 24-70 II has over the 24-105, especially at the edges. Wide open at f/2.8, it is sharper than the 24-105L was at f/8... Not even exaggerating here. It is sharper at every focal length and aperture, compared. If you love extreme detail and sharpness, the 24-70 II is a no brainer.
The 24-70 II is really like having f/2.8 primes from 24mm to 70mm... I always enjoyed my 24-105L but it never did "wow" me. It was an excellent lens that way good at everything, but did not excel at anything. Very good walkaround lens, no doubt. The IS is nice, build is great and the performance was very good.
I don't miss the IS one bit, honestly... The sharpness/build/performance WAY makes up for the loss of IS... At least for me.
IMO, the 24-70 II was VERY MUCH worth the upgrade, for me, over the 24-105L... I think it justifies the price, personally. I like it better than my 70-200 II, but that is not exactly the most fair comparison