So first off, don't take my word on it, without buying the book you can look up all the rules for free (and legally) at the Archives of Nethys (link to the Beginner's Guide).

In short, while Pathfinder 1 is indeed "D&D3.75", with some fixes to some elements that break in 3.5, it's mostly more of the same and a lot of PF1e players use(d) 3.5 material to add to their games. It was still enjoyable like 3.5 but if you already own 3.5 stuff there's no real point in changing outside of wanting to change (because of WOTC being asses or whatever). They also later brought Starfinder, which is mostly Pathfinder 1 in space, with a couple new rules (mostly Stamina as a secondary, easy-to-recuperate, second health bar). Quite enjoyable also.

Pathfinder 2e is pretty different, but also similar. As others mentions, everything is much tighter ("every +1 counts", only 3 possible types of bonus), so you will not have munchkins breaking the game and forcing an arms race on the other players and the DM; and also, the encounter calculator works! As a DM, it's much easier to prepare for. They kept the large customization core, which differentiates it the most from it's main concurrent D&D5/5.5e, so you still have at least one choice at every level up, usually two. As a player, I really enjoy how you can make the same class/ancestry (race) combination feel very different. And there's like 28 classes now. Technically even more since Starfinder 2e is also out and is mostly cross-compatible.

There are of course some weak points, and I'll gladly expand upon them and any other points it if you're interested. But I've really enjoyed my time in PF2e on both sides of the screen and consider it my TTRPG of choice currently, even as a 3.5 Grognard.

That's the whole point. Pathfinder's origin is a collaboration of people looking at DnD4 and thinking "we can do better"... And for quite some time they were right as DnD's 4th edition was really that unpopular.

2e is well made, and if you read tabletop systems for fun it’s worth knowing in passing, but it’s not going to inform how you play other systems the way Blades in the Dark or Call of Cthulhu might. Half the core book is conditions, traits, and spells.

I do encourage people interested in modern fantasy games to learn it to play it. For gamers who want crunch I much prefer it to 5e. Paizo’s #2 for a reason.

As far as I know, Pathfinder 1E was a continuation of D&D 3.5 after they moved on to D&D 4E. I think Pathfinder 1E is compatible with D&D 3.5, but don't quote me on that. Pathfinder 2E is a much more tightly balanced system, and is probably best for its 4 degrees of success, where 10 over a DC is a critical success and 10 under is a critical failure.

Also, Pathfinder has way more options to customize your character as you level up compared to D&D 5E.

Purely customization. Long-term players of 3.5 started doing wacky multi-class combos to make their character more unique. WOTC chose to keep it simple with 4th ed and i think successfully streamlined the rules in 5th. Pathfinder chose to add greater customization to each player class so that while multiclassing is still an option, a straight class PC can still be quite distinct.

Honestly, as someone who was all for a new contender in the rpg crawl/slog category, PF 1.0 certainly seemed like D&D 3.75. (and it didn't hurt that a number of OGs had left D&D to help launch it.)

Apparently, many felt that it was better at D&D than even D&D at the time, though I've since become an indifferent fan of it at best.

Same slog, same crunch, same vast array of "options" without much in the way of lowering the myriad barriers to entry, etc. Meh.

Good thing now there's Daggerheart, Blades in the Dark, Powered by the Apocalypse, Worlds/Stars Without Number, et al! 🤩🤘🏼