While not what people were expecting from a sequel to Quake, it still keeps to the Quake roots, and I enjoyed it very much.
Quake II adds an actual storyline to the Quake series. In this game, you're fighting against the Strogg, once again all by yourself. The levels are objective-based, however they still play pretty much the same as original Quake levels, but less maze-like for the most part. This may put some people off.
The game has been given a complete graphical overhaul; more or less every texture and model in the game has been completely remade to fit the look of Quake II.
I would write more, but I haven't played this game in a long time and my memory of it isn't too good. What I will say, though, is that it's still a fantastic shooter well worth your time. It's not what you were expecting, but it's a good game in it's own right.
Oh, and yes, multiplayer is still there in all it's glory, but it still uses the direct IP method of the original so you must scavenge the internet once more for servers that are still alive