I started breaking mine in this weekend, did about an hour on a treadmill at incline, then lifted weights in them on Saturday. They felt good, no issues, so I bravely put them on my feet for a 8 mile hike on Sunday with +3k feet in elevation gain and they were perfectly fine. I did have to re-lace about 3 miles into the trip, but then never touched them again. I was really impressed on the downhill, my toes never hit the front of the boot, and on some super steep slippery stuff they handled it beautifully.
To answer the stiffness question...They aren't as stiff as I thought they would be, but they are pretty stiff. They walk on flat ground great, but do great on steep as well. They are stiffer than Crispi Nevadas, probably close to Iowa Tibets from what I remember (haven't worn the Lowa's in 2-3 years).