Just how good is Astolfo? worth building up?

He's better than Kintoki (Rider) for farming, and has buffs coming up fairly soon to make him even more useful.
I personally use Zerkerlot/Atalante/Astolfo for daily runs.

His third skill becomes a solid NP battery, making him an equally solid farmer not unlike Medusa. He still suffers from the issue of having low base NP gain and crit star generation in spite of having three quick cards but getting that three-hit evade on his noble phantasm can be useful for stalling things out in certain quests.

I have beaing spoked by him 2 time, and becouse i dident really like the servant, i make my reserch on him to see if he was worth the investment (i had no gold rider back then)
Short answer... no.
He is basically a weaker Medusa, who is a 3 star, in the Friend pull, easy NP5....
2 out of 3 skill are the same but better in medusa case.
He have a stun with lower stun chance, and longer cd.
A weaker steroid thet last 1 turn instead of 2 like her's.
his third skill will be his rediming point once the buff come, but for now, is .. unusable. potentially a creazy good skill, but low chance of proc on 3 different buff and he need all of them to do something. Sure, 50% np battery is going to be great, but medusa have a 20%, and for farm is good enought most of the time., plus he even need to level thet skill for the extra np...
His star and np gen is awfull, and his base stats are bad to...
Those 2 have a similar np too......
the only thing he have compared to medusa are the 10 level and the 3 hit evede on np, on a servant used only for door farm....

I actually belive he is one of the worst servant in the game.

Better level medusa.

Damn that's a lot of SQs, if you want to ease some of your salt then level Astolfo bring her to the backline or as your daily farmer and he's usable.

Gameplay wise, Probably he is one of the most inconsistent servants in the entire FGO. His kit : Stun, RNG based instead of 100% chance, Atk Up only 1 turn, RNG again for all of 3rd skill buff, all of the buffs RNG based with 65 % chance for each turn.

Just bring him to a berserker infested enemies and it's totally a walk on the park. He loves bullying and eat strong Zerker bossess or enemies.

Personally, i would raise him as a reminder of how bitter Gacha is. At least he could be your Quick Last Man Standing if your Chu is max Bond or another anchor is maxed.

His price worth more than 600 Saint Quartz, don't burn him.