Well, I played my first two games of 8th edition at the store today, as they have preview copies of the books for use.
The mechanics are really straightforward, but plays the same as 7th. It is a bit weird charging with vehicles, but I really like the tactical uses of that.
We played 1000pts using Patrol Detachments, so no command points.
Building an army was really easy, all the points for all units and wargear was on one page, so no need to flip through pages.
I brought my Harlequins, and my opponent ran Necrons. One thing we found, is that 1000pts doesn't fit as much as it did under 7th. Also, my Harlequins are way more durable. Their Inv. save is one point better, and the Shadowseer gives a great defensive buff to nearby units.
Moving and shooting is pretty much the same as in 7th, but because you can always move and shoot, the game is a lot more dynamic.
Assault was a big deal. I guess less so for our game, as my speedy Harlequins always got to charge first, but when he was able to charge me, him going first was a big deal. Necrons are scary in combat now that they all hit on 3's.
I didn't think about it before playing, but inv. saves are amazing now. The ability to ignore AP is crazy good. Necrons shooting has surprisingly good AP, even on basic guns, and my whole army ignored that. However, his wraiths punished me as they did the same thing.
Looking forward to playing more games, but I really enjoy it. I'm a huge Age of Sigmar fan, and 8th edition 40k is very different from that game.
Looking forward to my pre-orders this weekend.