The Canoness is the linchpin HQ of the Sisters of Battle Codex, out of the two generic HQs you can get to lead your Battle Sisters, the Canoness is the more well known. Let take a look at her stat line
WS BS S T W I A Ld Sv
5 5 3 3 3 4 3 10 3+
Not really that impressive pretty much just your standard Sister of Battle with +1 to Weapon Skill, Ballistic Skill and Initiative. There are very little wargear options for the Canoness too and her save can't get any better, although you can give her a Rosarius so she has an Invul. at least, but like all Battle Sister, she already has a 6++ due to Shield of Faith rule.
As far as special rules go she has the usual Sisters of Battle special rules and of course Independent Character, also Stubborn, which is kind of useless except if she is leading a squad of 20 Battle Sisters, which is where she should definitely be. Her Acts of Faith power is great, but as a counter, giving her and the unit she is in Preferred Enemy and +1 to Initiative. She also unlock the Sororitas Command Squad, like a Space Marine Command Squad, but with less options and being Sisters of Battle.
All in all, the Canoness is no mighty hero, she is a support character that buffs a unit and should most likely be used that way, mainly with a Sister blob so they never fall back and it also makes the units Act of Faith test work more often.
The second HQ is the Ecclesiarchy Confessor, now I've never used him and there are many reasons why. Firstly the stats line is the same as the Canoness only instead of a 3+ save he has a 5+, though he comes with a Rosarius it still doesn't make up for the fact he has a terrible save and costs more than the Canoness. His option list for wargear is a little longer than the Canoness and he can sport a Plasma Gun which is quite awesome especially with his BS of 5. He has almost no special rules the list is this. Acts of Faith, Independent Character, Righteous Rage and Shield of Faith.
Shield of Faith is pointless to have since he has a better Invul. save anyway with the Rosarius, he has no Acts of Faith to call upon, the rule is just there so he can buff the rolls of other squads and Righteous Rage is just like the Space Marine Chaplain's rule that allows re-rolls on the turn the unit charged, kind of good but in an army that doesn't have a lot of assault units, it isn't that useful. He does unlock the Battle Conclave, which is essentially an Iquisitorial Henchmen squad with only Death Cult Assassins, Crusaders and Arco-Flagellants.
I would only play this guy to buff a Battle Sister blob and to have that Plasma Gun.