Okay, I love the F1's...don't get me wrong. For all-day touring (I don't rando race, or anything), paired with a light, soft-ish ski like the old Voile Carbon Surf's, these boots simply cannot be beat. The vertical feet just rack up, since these boots are extremely light. Just be warned: you will likely have to completely change the way you ski, since these boots offer little fore-aft support on the downhill. Not only will you have to be more balanced fore-aft, but left-right as well. You have to be much more "centered" on your skis. It took me a couple days in the BC to get used to it. Once you get the technique down, these boots are a real joy to use. I haven't tried pushing big, heavy skis, but I can't imagine it'd be very easy.

I recently went to a F1, Dynafit Vert ST, K2 Sahale/Rally set up for light/fast touring and fitness laps at the resort (lots of vert, well at least for me...). The F1's are exactly what I was looking for: lightweight, comfortable, awesome on the uphills, and perform decent coming down. I have a fairly wide forefoot, med high arch and a narrow heel. They fit me really well. I've skiied them on hardpack, soft groomers, spring slush and powder, and they surprised me how well they work in all conditions, although I feel you have to be more deliberate in your skiing, that is, they take some effort to perform well. But the trade off between being superlight and comfy on the uphills is well worth a little performance loss on the way down in my opinion.

Coming from a light Telemark background
I find this is all the boot I need in the backcountry. Paired with Trab freerandos it is a match made in heaven. Learning to operate the latch mechanism efficiently takes some practice and I do worry about durability, but have used it for 2 years without any problems. Just have not found a need for any beefier boot for most the backcountry skiing I do.
Also have a F3, but find I spend most all my time in F1's

My experience with F1s is limited to 6 long day trips in the Sierras including varied snow, booting with crampons, and rock scrambling. The fit: liners are better than other brands I've used. No blisters, which is surprising considering how little control there is to tighten the liners in climbing mode. Liners are toasty. Performance: F1s really shine on the uphill. Their skinning efficiency is amazing! Bellows rock for skinning! Scrambling was a bit awkward because of the stiff tongue, and takes getting used to. Not sure if the bellows add advantage there. Some crampons won't fit because of the protruding part on the back (the light camp fits!). Skiing is basically as described by most people.

This pair is for my wife, I've had a few pairs of the F1 Ski Boot- they keep getting better. An excellent touring boot, very comfortable, pretty light weight and easy to transition from Ski to walk and vice versa. The bellows makes climbing easier- the steeper the climb gets the more it helps. The bellows and open tongue allow for a long stride with out the chaffing of most boots.

I've skied Alpine for 40 years and this is my first backcountry boot. It worked really well, both up and downhill. I got them fitted at Footloose in Mammoth Lakes and they did a great job. As others have noted, you have to stay on top of your ski, but in my opinion, that's the best way to ski anyway. I'm a big guy ~250 and was concerned these wouldn't be enough boot, but they were fine on intermediate resort and backcountry runs. I'm confident enough to try steeper runs in them.

I use the F1 for serious tours and rando races. Very light, comfortable, etc. I sometimes have a hard time getting the boot to lock into downhill mode. I'm hoping that once I break them in a bit more, it will become less difficult. I also had the tongue pop out of the holder, but found it pretty easy to get back in place. I prefer the Scarpa Spirit 4 for day to day skiing when I'm less weight conscious, but overall the F1 is a great boot!