With the arrival of Steam, Humble Store, GOG (few months away), Desura and many other outlets serving games for Linux and specifically Ubuntu and its derivatives, I believe that the Ubuntu platform is ready for use as a gaming OS.
Some numbers: The Steam Linux library has grown to 484 games for Linux. The Humble Store has around 260 games for Linux. Desura has 556 games for Linux. The Wine HQ platinum and gold ratings have 3874 games.
Some recent games that are confirmed to come on Linux: Metro 2033, Metro Last Light, XCOM: Enemy Unknown, The Witcher 2, Stronghold 3 Gold, Wasteland 2, Euro Truck Simulator 2, Civilization V, and many more.
In addition, nearly all games announced on Kickstarter and other crowd funding platforms are coming/already are available for Linux. Games like Wasteland 2, Pillars of Eternity, Road Redemption, Shadowrun Returns, and many more crowd funded games support Linux.
Yes, some big games do not feature on Linux yet, but many of them are coming in the near future. For these, you can use Wine or PlayOnLinux (depends on Wine) or CrossOver Linux (from the supporters of Wine. This is a commercial software with excellent support for Windows games. And, it will cost just a fraction of what you will pay for Windows).