I like it personally. Always have liked the whole series.
The games get better with more enemy variety, better writing, level design, visuals and core gameplay too. But fundamentally it's the same loop as in the first game. I do however think the 2nd, 3rd and pre-sequel are much more narrative driven. So that's appreciable if you like the story and tone of the games (even if they are hit and miss depending on the game and questline). So today the first game does come across as quite simplistic I suppose, but I still like what it's doing and comparing it to other shooters from its era I think it's a very interesting game.
The gameplay I think it works more when you have people to play with, because you tend to decide on a build and weapon preferences and you're sort of helping each other as you go through. Maybe one of you is a sniper and one of you is more of a tank and you're playing off each others strengths and weaknesses, also trying to see who can go further with their build faster, and break the game the most. I think that's very entertaining. Looting and exploring with a friend is a lot of fun as the games are littered with secret chests to find and lots of optional routes and objectives.
I know the series gets a lot of flack, but I like it a lot personally. There's also a lot of good representation in the series for groups that tend to be underrepresented in the industry, such as LQBTQ+. The game missteps sometimes, for instance it has some enemies called 'midgets' and it calls others 'psychos' and things like that. But on the other hand it does have characters with disabilities too and characters with lots of different body types, I think for a product of the games industry where games are generally really not very diverse at all, the Borderlands series actually gets a lot more right than it does wrong.