Grant Morrison was one of the first comic book authors I got into as a teenager, back in the late 90s/early 00s, and I think he’s fantastic. I’m not a huge comic fan, so there’s still a lot of major works of his I haven’t read, and I don’t have the same breadth of knowledge of comics in general that other fans on here have, so I can’t speak with much authority about his influence. But I can say that he’s got a real penchant for mind-bending, often downright psychedelic stories, which are probably my favorite type of stories. The first series of his I read was The Invisibles (this would have been after I had started getting into comics because of The Sandman, and had read some of Alan Moore’s stuff like Watchmen and Saga of the Swamp thing), and it blew my mind. I think his run on Doom Patrol
But I never really had the money to buy a lot of comics, and didn’t have access to a library with a good collection, so it wasn’t until relatively recently that I realized that he’s also known for writing “normal” superhero comic books like JLA and Superman, though from what I’ve seen there’s usually at least a bit of weirdness even in those runs.
I’d also second pretty much everything @darkstarz said: you can really tell that Morrison has a deep knowledge and love of comic history and it makes his stories on long-running titles that much better, and he also does seem to think quite a bit of himself. This can lead to some of his stories just having way too much stuff going on, and you might finish something and not really be sure if you fully understood what happened. I’m still pretty sure I don’t totally understand everything in Final Crisis. But on the whole I still had a lot of fun reading it.
Also Supergods is great. Definitely check that out.