Last night I had my first game of 6th ed 40k with my current IDM build of Necrons vs Capnwoodrow and a Chaos SM / Demon ally list. The changes in the way you have to move your units and the changes in the assault phase will take some time to get use to. We didn't have any issues really with the new wound allocation shenanigans (though to be honest we didn't have any Character units, and very few mixed save units.) In the end because I have very little experience against Chaos, I didn't focus fire the right units and lost in the new kill point mission.
The first thing I noticed was that the fliers, while hard to hit, are not invincible. Cap got 2 solid shots on 2 of my fliers, and both ended up being destroyed. One exploded and that was kinda cool, and the other got stunned and then couldn't move 18" and wrecked 1 inch away from a plague marine squad. The flight rules for movement and the Necron 36" & disembark will take some getting use to.
The second thing I noticed was that shooting feels better with the change to closest to furthest wound allocation. We didn't have any units that would cause the wound shifting shenanigans or a mixed armor save so I can't comment about how those will affect my initial thoughts.
Finally the assault phase. Where to start? It's going to be tough to remember not to jump in the movement phase to get the re-roll / I10 hit in the assault phase with jumpers. They need to FAQ chariots a little in combat. Not a 6thism, but I need to remember to not assault a MC with an Overlord with no invuln.
I had fun in the game, and think the game could get interesting in the coming edition. I'll be honest I find the possibilities with Allies, Fortifications and even Dual FOC will be interesting to mess around with, but I know we probably wont see Fortifications and dual FOC in national events past Feast of Blades (or even Nova) this year.
Because of the FAQs Allies aren't as broken as everyone was fearing before the release of the rulebook.
I partially agree with Jester that some of the fortifications (namely the sky pad and the fortress) are too large for tournament tables ( pregenerated table terrain. ) I think the bastion and the ageis would be ok, however I think TO's will find it easier to just say no forts than to introduce complexity in list building.
The Dual FOC @ 2k will probably just be rules lawyer-ed away by TO's saying that their event is 1999 points from now on. I could even see events possibly being knocked back down to 1850.
All in all I think 6th will be good, but until they release the BRB FAQ it's going to be confusing on how some stuff works.