I played my first game with the new sisters, won against nids which was a close game.
I have found out in a 2k game 5 to 6 faith a turn seems to be enough. Most units midgame pass faith +2-4+ making the 20 point upgrade to re roll faith a waste of points.
The seraphim I ran with 2 hand flamers which really tear up infantry units. The re rolling to wound aspect is really nice, so shoot 18 times, and get 20 wounds on a flamer, and if they are still alive assault with 3 attacks a piece at ws 4 and they would not be alive. Celestine works well for the added faith, and close combat support. I put in an eviserator now so it will help out more in combat. Also the re rolling +6 invul save does help the unit out.
I know working on my dominion squads with 4 melta and a plasma pistol. The re rolling to hit can help with that 1 or 2 plasma rolling, and can tear up units like plague marines.
The standard troops rerolling on ones is also nice when rapid firing. It's almost like they have bs 5.
The +6 invulnerable does help. I forgot my vehicles have the invulnerable, but still it help saved a few troops here and there.
Also the Repentia have worked out well too.
I also wanted to point out that a squad of faithful sisters cost 124 pts while in this book it cost 125 pts. If you take a flamer and melta the squads cost the same. If you throw in a rhino you paying 18 points more in the old book. With the new dex for 18 points less you're getting re rolling on one's, the abilty to re group, frag, krak, and a +6 invulnerable. The squad can now take out low armour vehicles with the krak grenades. My main complaint is that they can not take an Eviscerator.