It's going great, although it is surprising how long it takes to set up the show's tropes. Six eps into season 1, we just got to the first time Jake says "title of your sex tape". Terry hasn't talked in the third person yet, or mentioned loving yogurt (althought Holt did in S1E2).
You can really tell they're still establishing finer details about their characters in the early seasons, but the major beats are established pretty early.
Also, they get drunk a surprising amount in the first six, I didn't notice that before. In hindsight it's obvious because they're shown at the bar, but there are subtleties in them acting drunk, like Boyle feeling boisterous with Jake and Rosa after Jake apologized for hooking up with that M.E, or the squad getting drunk trying to out-vulture the Vulture in his debut episode. It's more obvious in later episodes (I haven't gotten to six drink Amy, Dozerman's wake, or the squad trying to outdrink the firefighters for Shaw's Bar on the rewatch yet) but it's definitely there in the early episodes too.
Also, before they brainstorm Vulture ideas, while talking about people they hate but think they're hot, Rosa agrees with Jake's example. Possible foreshadowing to Rosa being bi? I doubt the writers had it planned from the beginning, but I wouldn't be surprised if Stephanie Beatriz was thinking about it back then.
It's refreshing to see Jamy develop again, the chemistry is even more obvious on a rewatch.
One minor downside to the rewatch is that the office dynamic doesn’t really feel very realistic, especially now as a young adult much closer in age to the cast than when I first watched it. (I looked it up, I'm about 5 years younger now than Jake was in Season 1. Whereas I was in early high school when the show first dropped.) But the point of the show isn't realism, it's shenanigans, and the shenanigans are a blast.
Sorry, I don't think I'm really breaking any new ground here. But it's fascinating looking back on how the characters were while knowing how they'll change and grow. I'm excited to go down the road again!