MGS2 takes the controls of MGS1 and adds to them, vastly improving the gameplay (camera still sucks, though, and is monstrously bad without the radar). You can shoot in 1st person, hang from railings, swim through flooded corridors, hold up guards, press your back against walls and jump out to take potshots. Your inventory is varied, but most of it alerts guards and therefore goes unused. Once you learn them, the controls are generally intuitive, one or two exceptions aside (swordplay controls, cocking your rifle versus firing it, etc.).
AI is more complex but still has its oddities. Guards do sensible things when they spot you, and are sometimes indecently skilled in flushing you out of hiding. Then they do nothing when one of their number vanishes, even if they investigate it. They're still curiously myopic, too. However, there's no denying most of the fun is in how you deal with the guards and their surroundings, since there are dozens of options here, and each area usually throws a gimmick at you that promises more challenges and more ways to be cruel or sneaky.
Level design tends to the narrow and enclosed, and few areas are gigantic, so the game seems to fly by unless you get engrossed in guard torture or distracted by the breakable scenery (pretty and impressive, but ultimately superfluous). Unfortunately, the game makes you backtrack early on in the Plant chapter, and overall seems pretty short despite how huge the Big Shell and Arsenal Gear should be. While the Plant's level design and artwork is still good, it simply lacks the atmosphere and awesome grittiness of the Tanker section.
Sadly, the gameplay feels like a sideshow to the cutscenes (and codec calls, irritatingly), which is a shame because the plot can be best summed up as "Kojima's progressive mental breakdown". It's not incomprehensible, per se, but what starts out as a promisingly atmospheric MGS sequel quickly delves into characters playing Top Trumps Gambits at each other, an unenjoyable and groan-worthy conspiracy, overdone Fourth Wall Breaking, a flood of cutscenes, and basically Kojima using pretentious philosophy and the interface to mock, confuse, and show off at you, generally pretending to get you questioning reality rather than the writer's sanity. The supernatural elements don't help, either.
Overall, worth getting. Just skip the cutscenes.