It's quite nice. I watched the second season too. It's a comparatively low-stakes slice-of-life isekai, but it's a pleasant way to spend a few half-hours.
Sei is a Japanese office worker who's been summoned to a fantasy world and given amazing magical powers! No one knows this yet, but it's true. It's even in the show's title. Sei will eventually be acknowledged as the legendary Saint and will amaze all and sundry by creating mysteriously powerful potions, casting healing magic and using legendary magic that's impossible for the country's strongest magician. And will also be surrounded by hot but deferential members of the opposite sex.
What's different from the standard isekai template is that Sei is female (like the author of the original light novels and very probably also the target audience). She has magical superpowers, yes, but she doesn't have battles or go on quests to defeat a Dark Lord. She researches herbs. She improves Salutania's cooking. Her main roles are "potion maker" and "medic". She goes on expeditions with the knights in the second half of the season, but basically she's a nice girl who's found a job she enjoys and to which she's well suited. It looks quite pleasant being a herbalist in the Royal Research Institute, especially when (like Sei) all your spells and potions are 50% more effective than the same magic from a native Salutanian.
This is relaxing, but it's not outright slice-of-life. Another Japanese girl called Airi was summoned at the same time as Sei, so Prince Kyle insists that Airi's the Saint and barely even acknowledges Sei's existence. (He's less of a moron than he seems, but for a while you'll think he's terminally boneheaded.) Then there are the monster-hunting expeditions, on which most of Salutania's knights would have died if Sei weren't around.
There's some understated possibly-maybe romance, although it ultimately becomes plot-important. Season 1 ends at a point in the original novels where nothing had been said or resolved... but its last episode tries to make a small something out of nothing anyway. This is negligible and not particularly successful. At this early stage, at least, it's probably a bigger deal that Sei has lots of handsome, obedient and powerful men at her beck and call. Even someone called the Grand Magus will be young and sexy. "How did I get surrounded by gorgeous guys?" she thinks in ep.5. (Hell, even the king is handsome and doesn't really look older than anyone else, despite having adult sons.)
It's pleasant escapism. I always enjoyed these episodes, although there was never anything particularly urgent about them. Sei is modest (e.g. trying to maintain her cover as an ordinary person as long as possible), thoughtful (e.g. her acceptance of the king's apology) and kind (e.g. trying to ensure a happy ending for Airi and for that girl who was embarrassed to go to school in ep.2). She heals people. She's polite and nice, although a bit of a disaster area when it comes to romance. She's completely thrown by the idea of a man showing interest in her and she denies in ep.3 that her date is a date.
Would I recommend this show? Maybe, sort of. It's not drama-heavy and you won't think it's amazing or anything, but you'll probably enjoy it.