These guys are insane, same stats as a Battle Sister but with no armour save, except the Mistress of Repentance which wears Power Armour, and 2 Attacks. Now these sisters have some pretty cruel weapons as each Sister Repentia is armed with an Eviserator which is pretty deadly, not to mention the Mistress who carries two Neural Whips, which are Power Weapons that give her a Strength of 8 but you roll to wound on the enemies Leadership profile. Wargear aside, what pushes the Sister Repentia squad into the area of greatness is the giant list of special rules they have.
Feel no Pain, Fleet, Fearless, Rage, Shield of Faith, Acts of Faith and Spirit of the Martyr (which is their Faith Power) this impressive list of special rules definitely make up for their lack of a 3+ Armour save. Spirit of the Martyr is their Faith power and it rocks, you take your Faith test at the start of the Assault phase, if successful, when a Sister Repentia is killed, do not remove the model, each fallen Sister Repentia gets 1 attack at the enemy before they die! its great, because if your unit get completely wiped they can still hit and I mean hit hard.
Sister Repentia are good, I use them quite a bit but they are quite often hit or miss, having only a 6+ Invul. and Feel no Pain to keep them alive they are constantly shot off the board quite quickly and Sister Repentia aren't cheap. It is a risk vs. gain scenario and you just have to take the risk sometimes, because when it pays off, it pays off big.