I've been down this road for awhile now.
Gateway drug to hardbooting was the M6 Phantom bindings. Loved them so much that I ended up riding them all the time in the resort. Worked great for awhile... until the metal pins began wearing down the metal cleats. The rocking motion of the pins over time ends up drilling straight in to the cleats until there's just too much play in the system.
I'm currently on the Karakoram HB Guides. They've been working well so far, but have had some problems with the way that the rear heel bails are formed. It's not a circular form like in F2 bindings or the M6, but are more... angular. During my few days on them, they've caused noticeable deformation to the rear cleat of my boot. Time will tell if this becomes a bigger issue, but in the meantime I'm investigating alternative options of how to get the bail to fit better.
How have the F2 Races fit out of the box for AT boots? I have a Carve RS that has a rear bail that seems way too high, but maybe I'm missing out on a really obvious adjustment.