Gran Match is by far the best pool hall in Madrid. It's a place that emphasizes relaxing get-togethers among friends to shoot some pool, play poker, watch football on TV, or just hang out and have some drinks. While there are more centric pool halls in Madrid, they're also more expensive and a lot more crowded. Gran match has roughly 20-30 (or more) American and British billiards tables so there's always a place open.
They have a full-bar so you can grab a beer or even a snack while you play. It's open until about 3am on weekends and has a pretty good atmosphere after midnight. The have a section of tables where local poker fans bring chips and cards and set up to play quiet games among friends.
The table isn't expensive. We were playing for about 2.5-3 hours and the bill for the table and balls was around 20 euros. All in all, it's a really recommendable place!