The stats of a Battle Sister is this:
WS BS S T W I A Ld Sv
4 4 3 3 1 3 1 8 3+
Pretty much a Space Marine with reduced Strength, Toughness and Initiative which is ok for a model that costs 12 points. Having Power Armour saves these girls a lot of pain, especially when they are susceptible to it due to low Toughness and the Shield of Faith rule gives them a 6+ Invul. Save giving them a slim chance to avoid dying to AP3 or lower weapons. Armed with Bolters, Frag and Krak Grenades and a Bolt Pistol, Sisters are pretty well equipped to begin with and the base squad size is 9 Battle Sisters and 1 Sister Superior with the option to take an additional 10 Battle Sisters meaning you can get a pretty large squad of Power Armour wearing troops. No matter what the squad size is only two Sisters in the squad can take special or heavy weapons and depending on how your Sister squad is designed to fight depends greatly on these weapon options, if your squad is a back line objective holder you'd take a Heavy Bolter and Storm Bolter or if your squad is an aggressive attack unit you'd have a Meltagun and Heavy Flamer.
The Sister Superior is pretty good for a squad leader, having an stat line like a standard Sister with a +1 to Attacks and Leadership. Her wargear options are really good to, being able to take Power Weapons, Combi-Weapons and even Plasma Pistols, she can really add punch to a squad by adding some extra Plasma, Melta or Flamers to your squad.
Special Rules for the Sisters are simple, Acts of Faith and Shield of Faith, as well as the Faith power the Battle Sisters have called Light of the Emperor. Light of the Emperor is one of the best Faith powers in the codex, if used in the Movement phase, the automatically re-groups and can act as normal, if used in the Shooting or Assault phase they re-roll all 1s to hit, this is awesome and thats not all it is also the only power that can be used multiple times in a turn, once per phase.
They are a pretty good Troop choice and being the only choice in the codex the do well to be as varied as possible to make up for the lack of choices and on more then one occasion it is my Battle Sister squads that win me games.