I own the first game. It was revolutionary for its time with using two joysticks to accomplish tasks, but it hasn't aged well. Graphics don't hold up even to some other PS1 games (slowdown; ugly textures; blocky character designs), and the music is either really forgettable or obnoxious. There are a few OK tunes in the game but nothing hummable or memorable in the sense of placing melodies to specific levels.
Also, I was never a fan of only being able to store four items in your inventory at a time, thus making it tedious to go to the menu and swap during gameplay. They should've just made the "change weapon" button Select, so you could cycle through all of them on the fly.
The monkeys can be a pain to catch, thanks to the not-so-forgiving hit detection on touching them with your net and their ability to jump around while you're not nearly as agile. In fact, compared to some other 3D platformers, Ape Escape feels slightly sluggish.
Then, of course, there's the replay value. Similar to the Spyro games, once you collect the various items in a level (in this case, monkeys), they're just not as fun to play through the second time around, as it'll be empty aside from the enemies and those triangles.
The camera sucks, too. Too often, you're either too close to the camera or it's obtrusive, and the camera re-adjust button isn't very helpful.
So overall, I'd rate is as average today.