I love this game, in 2 player games with my wife, I think it's about 18-2 to her, I'm not so good at the strategy!
The only bad thing is when in a 4 player game, depending on your placement of settlements and what happens right at the start of the game with road building, you can immediately find youself in for a very hard long slog and be stuck on 3-4 points knowing that you have no realistic chance of winning and are just waiting for one of the other players to get 10 points.
One nice thing about a game of Settlers is that even if you get locked out early, you'll only be losing for 45 minutes at most. The game is downright quick to play. Around here we go from opening the box to packing it up in about 30 minutes usually.
The only bad thing is when in a 4 player game, depending on your placement of settlements and what happens right at the start of the game with road building, you can immediately find youself in for a very hard long slog and be stuck on 3-4 points knowing that you have no realistic chance of winning and are just waiting for one of the other players to get 10 points.
3 players is the optimum I find.