I'll be honest: I completely obsessed with this game back in the day, and I've lost track of how many times I've played it through. I haven't had an N64 for at least a decade, so to see this on GOG was pretty exciting. I sat down to play, still remembering all the secrets and where to go to find every little thing, thinking this would be nice to finally have again, but I was still surprised. Playing this with mouselook makes a world of difference; it's a HUGE improvement, and now that draw distance doesn't suck anymore, the game looks a lot better, as well.
This isn't going to be a game that everyone enjoys. As I haven't played a lot of modern Shooters, I can't really speak to the idea that it's dated, but I have heard as much. You'll also find quite a few platforming sections, and if that turns you off, I'd definitely stay away. I will say that the platforming is a lot better than in many of its contemporaries, but I can see why some would flinch at the idea.
Most Shooters of this era stand mostly on the merits of their respective arsenals, at least in my opinion, and this one more than delivers. This game has a ridiculous number of weapons, many of which are quite creative; it wasn't afraid to stray from the melee/pistol/shotgun/machine gun/weak energy/powerful energy formula that Doom had popularized. In fact, it has more than a few that are so over-the-top that even the BFG would turn its face sheepishly away and furtively slink out of the room.
So, if you're not one to shy away from a little first-person platforming, and think that taking down cybernetically enhanced velociraptors with a nuclear-apocalypse-launching energy rifle, then give this one a go!