Conclusion: If you enjoyed Enemy Unknown and are ready for more, XCOM: Enemy Within's additions and alterations make it a must-buy.
Pros: Adds a ton of new content that alters the feel of the game while still retaining its better aspects. Teams of augmented operatives, once powered up, can be empoweringly devastating against even the strongest aliens.
Cons: Increased frequency of UFOs and alien abductions means you often won't have time to prepare or recover before the next attack. The surprise base-invasion can effectively end your game if the random operatives it picks aren't powerful enough to handle it.