I play World of Warcraft and it runs very damn well. Now, I can't compaire it to an XPS or anything. (Funny thing is, that WAS the machine I was going to buy before I bought my MBP)
Anyway, I get anything from 20 to 60 FPS depending on the area. 40 people standing around you, plus 30 mobs on a BWL raid and I still get 20 FPS. Which is very playable.
Keep in mind, thats with all settings maxed.