PokerStars is the largest online poker site in the world, with plenty of cash games and tournaments offered 24 hours a day. It is one of the few poker rooms that is not part of a network of many different sites. That is, all of your opponents are also playing on PokerStars client software for mac, PC, android or iOS. Although they also bought Full Tilt poker, each site remains on a separate network.
Software quality is extremely high, with clean graphics and logical navigation. Advance action buttons and smooth animations make for a fast game, and multi-table play is well supported. PokerStars offers poker software for mac, and you can also download PokerStars for Android and iOS, allowing you to play on a smartphone or tablet.
PokerStars also has one of the most recognized poker VIP programs, which is ironic since it offers poor rewards for anyone who generates less than $100,000/year in rake and fees!
Despite needing to be 18 years of age or older to play on PokerStars, the poker room has very strict chat policies and moderators routinely suspend chat privileges for silly reasons. If a moderator is called, there is no investigation - they typically grab the last line of chat entered and suspend the chat privileges for that player. Poor customer service at PokerStars extends beyond chat moderators. Most emails get lazy copy & paste form replies, which often have nothing to do with the original question/comment.