Quake II tends to get a bad rep. This is mostly due to the fact that it's a follow-up to one of the greatest FPS ever released, which is Quake. Quake II lacks the Lovecraftian world, is much slower, and nowhere near as frenetic and challenging. Once one has accepted that Quake II is a very different game, one can begin to like it for what it does well. The enemies are varied, encounters nicely balanced, weapons progression makes a lot of sense, and the levels manage to be semi-open yet you still always know where to go and what to do. The only real issue are the bosses: You pop quad damage and that's it, each of them takes 5-10 seconds to beat. Also, if you don't like brown or gray, the graphics may not be to your liking.
Each of the mission packs are roughly half as long as the original. The Reckoning essentially is more of the same, albeit not as inspired. New enemies are just old enemies with new abilities, the levels can get confusing at times, and some of the new weapons are game-breaking good. The last few levels get really tedious, as you're fighting an enemy capable of one-shotting the player over and over again.
Ground Zero is usually panned, but for no good reason. The first Unit is clearly the weakest: overly complex and confusing, with lots of backtracking and 'What now?' moments. From there, it gets better and better: Challenging new enemies, decently original weapons, and some imaginative level design. Sure, the turrets are tedious, but two rail hits and they're scrap. Some have said Ground Zero is unnecessarily hard, but even on hard difficulty, it really still isn't. Bosses can still be vaporized within seconds by abusing power-ups.
Overall, while not as memorable or genre-defining as its predecessor, Quake II is a straightforward and fun little FPS that aged surprisingly well.
I recommend playing this in a source port. I've used Yamagi and had zero issues at any point.
