Well, I have some experience of both IL2 and CFS3 now. CFS3 has 5 DOF - all six minus roll. Having all those DOF's is great with my TrackIR4! Where I find it particularly helpful is where you can move your head around to peer around canopy stanchions. It is REALLY cool to lift yourself up out of your seat (RL) to peer down over your cowl so you can see if there are any targets hidden there. It really adds to realism and immersion!
One thing you are all going to have to get used to all over again is using your gun sights with 6 DOF. This is pretty danged hard! You can be slouching in your seat a bit (RL) or over to the side a bit and your sights will be way out! You have got to line your head up with something in the "office" in order to align your eyeball with the sight correctly! It turns out that for the crude WWI gun sights this is no problem because they have a distinct forward and aft sight so you can easily line your head up. (If you want some great WWI action, get CFS3 and install the Over Flander's Field addon! Its the best WWI action out there! See http://off.oldbrowndog.net/)
So, what I might hope for is for sowbob to have some sort of head alignment mechanism - something to tell you when your head is correctly aligned such that your sights are as they are supposed to be. I've seen all manner of workarounds for this the best of which is to put in a dead zone in the middle of your TrackIR response curve so you can settle in the sweet spot.
This is no-kidding a real challenge, fellas! Your scoring is going to plummet when you switch to 6DOF. My best is in a non-bubble cockpit, like an old Mustang, where you can use the structural stanchions all around to help align your head.
I do hope Saint Oleg is thinking about this!