pretty good, I don't mind going there for the next 3-4 years for research purposes. The crowds are pretty exhausting but the atmosphere in Spodek makes up for it. Obviously, prices of drinks and food in the area are through the roof, so it's better to actually go out and wander around Katowice a bit to find yourself a decent place. I had fun, the matches I've seen (CS - 1 quarter, one semis and a half semi, and final; SC2 - one semi and final; plus a bunch of exhibition games in weird shit) were pretty intense (maybe except the second map of the CS final). The expo in the MCK building is also a good place to visit, but I tried to avoid it, too many teenagers trying to win/collect gaming items for my taste.
I stayed in Gliwice, which I would definately recommend - prices in Katowice are tripled during IEM, so you can save up a lot if spending additional 30 minutes in train is not an issue for you.
8/10, would recommend, it's better to go out with friends and have some fun, though (thanks owca for showing up :P)