Most of the reviews I've read in the past covering board/card games usually set out a synopsis of the rules and then go on to comment about the game generally. It's unavoidable, but I will minimise my explanation of the rules and get straight to the review.
San Juan pits up to 4 players against one other in a race to score as many points as possible by combining the advantages conferred by the building of different buildings. I say "build"; there are no physical buildings or other components save for a deck of cards and tokens representing skill sets; builder, trader, prospector, governor and counsellor. A building is built by laying the relevant card in front of you and paying it's building cost by throwing away an equivalent number of cards.
The games allure lies in the game's balance; The designer has successfully created a game in which every decision is important because resources are extremely scarce, so the decision to build a particular building must really 'count', and because nearly every building available to be built carries a desirable benefit, this decision is hard. "Do I build the (cheap) Prefecture to get an extra card per turn? If so, I must throw away the (expensive) Quarry and forfeit the right to reduce the cost of buildings by one card".
There are a number of strategies that are available to use, and skill is needed to determine which strategy to use, based on the cards you possess. (There is no point shoehorning into your gameplan when you don't have the cards for it.) More skill is needed to know whether to abandon your strategy mid-game when another player undermines it, or you don't get the card you were hoping for.
The game plays extremely well as a 2 player. (I have played it for 3 - 4 years and am not bored by it yet.) It also plays extremely well as a 3 and 4 player game too. The game's duration is about 15 - 30m depending on how familiar players are with the game, cards and strategy.
It is easily learnt - either from the rule book, or (and this is best) from someone familiar with the game.
The downsides to this game? Some comment that there is little interaction between players choices. To the beginner, this is correct. Upon repeated play, subtle interactions are discernible which can throw the game one way or the other.
All in all, I would highly recommend this game to everyone. It is a family game. It is a game fro Christmas, for a games night, for the spare 1/2 hour whilst waiting for friends to arrive.
If you get one game and you decide to buck the trend of buying monopoly or other games that you might pick off the shelf in Argos or Sainsburys, then this is the game for you. It's really good value.