I think it looks pretty slow to start, and then has bad performance for an action game because it is too busy proving that their engine can render out a whole open world rpg map.
Serious Sam has always been a franchise of the formula where you run down a hallway, end up in an open area, and kill a hundred or so enemies, over and over for a dozen or so hours. I only played the first 3 levels of SS4 so far, but my experience is that it was more of that. I think anyone who's never played it before might hate it, but having played Serious Sam games since I was rather young, I don't mind it and it was really fun to me, with enough change from SS2/3 to feel fresh and different, without changing enough to feel different or to where it might have felt like it forced another play style on you.
If you liked previous SS games, SS4 is fun, and if you've never played SS before, SS4 is kinda just a mass enemy slaughter simulator without much challenge or skill, and if that's not your thing, it probably won't be very fun for you.
Serious Sam is the video game equivalent of a B-movie, and never has been more than that, and that's what makes it good.
I have played just after the Vesuvius level, and so far it's fun. Some things that I like more than SS3 are that the game starts throwing you more enemies earlier, and there are very few hitscan enemies (SS3 could get annoying at times in the earlier levels, which often were tight corridors filled with zombie soldiers). I don't like that you can't reload while sprinting, for some reason you could do that in SS3 but not in SS4, and it makes using the assault rifle very risky. Speaking of which, it no longer has perfect accuracy and suffers from recoil, like in a realistic shooter. Weird.
To sum it up, I haven't played a lot yet but what I have played I've liked. Some fans of the old games hated SS3 and this one is very similar so if you're one of those, you won't like the new one. Personally I've enjoyed all the mainline Sam games, so I have no problems with it.
Okay so I got farther into the game and I unlocked the ability to reload (and shoot) while sprinting. I wish the reloading was available from the beginning but, it's better than nothing.
There ARE pretty extensive shader choices for the renderer, including a vibrant cartoon mode with that weird "rich color" technique they use on the switch and in fallout 4 where the colors are "deep" somehow
Its rare to see a FPS which doesn't take itself too seriously and attempt to adopt shitty COD mechanics and pander to E sports. That alone is worth at least trying SS.