Jeanne D'arc

Yes. All 5-Star Servants are worth leveling, really, but Jeanne is in a rare position because she's a Ruler-class Servant. She'll take way less damage from the six normal classes (not Berserkers, though), and her kit comes with things to let the party survive what most enemies can throw out. She works especially well with Arts teams, and she'll get better once her second Interlude kicks in and she no longer gets stunned by her NP.

Jeanne can easily carry a low-level team, and at higher levels, she'll remain useful as a back-line defender who can finish a fight if your primary team can't. She's not good for crushing fights as fast as possible, so you won't use her for farming or anything - she shines when you CAN'T finish a fight too fast and need another tank to help you endure a slog. In a few fights, she'll also have type advantage against Moon Cancers, and it's nice to have *someone* for that.

Many people will point to her low damage as a problem, but that would be ignoring what her purpose is. Mash has low damage, that doesn't stop her from being useful. Jeanne provides a heal, an invincibility, a defense up, a stun that, while not guaranteed isn't terrible, some passive crit stars, and arts cards for arts chains. I would say her biggest weakness is the stun on her NP, but that is fixed (soon even, maybe? I know I looked up when to expect it but have since forgotten) with an interlude.

Just remember, Jeanne does get a strengthening quest down the line which does remove the stun on her np. Can you imagine that? A defensive beast with no stun to stop her from stacking nps in stall arts comps? Yes she is worth raising

Definitely. While her attack isn't that high, her NP will save your life when a boss is about to release an AOE NP that would otherwise wipe your team. On top of that, she is the ultimate tank. I'm pretty sure that she has, by far, the highest HP pool of all servants, and on top of that, being a Ruler, she is resistant to basically all classes (except Berserker and Avenger).