A Celestian is essentially a veteran Battle Sister and has similar stats expect for an Attack profile of 2 and Leadership of 9, also they are only 3 points more than a standard Battle Sister. This is a huge difference and for a squad thats max number is 10 I was a little skeptical on how they would perform, to my surprise they are actually quite effective. Wargear wise, up to two Celestians may take special or heavy weapons, similar to the Battle Sisters squad, though this just solidifies the fact that they are just glorified Battle Sisters.
Special Rules are the same as a Battle Sister squad too, except for their Faith Power, Hand of the Emperor, which give the Celestian squad +1 to Strength and Fearless in the Assault phase, wait, what? an Assault phase power for a unit that doesn't assault. Well, I guess you could charge with Celestians and really hurt what you hit, though I think the powers real purpose, like a lot of Faith powers, is defensive as it helps a unit terrible at combat become relatively good.
At best Celestians are expensive Battle Sisters, while they can be incredibly handy and can work well, I personally don't play them since there are so many better units like Dominion squads or Seraphim squads that do much more than Celestians. I'm not saying Celestians are a terrible unit, just that they need some improvments.