Depends on which game you would like to run on them. In the last five years even desktop video cards need to get their acts together to keep up with the latest trends, in netbooks your hands are sort of tied. You do not have to be without great titles, though, graphic is not everything.
My new and most recent favorite is Gemcraft Chapter 0. It's a tower defense style Flash game made not so long ago. If you've ever played a good tower defense take that and multiply the fun with two. There are half a dozen different gems to play with. You can combine them, throw them at monsters, use them in towers and traps to achieve different bonuses.
Most flash games work well on a netbook, but there is always an options tab where you can lower settings or press right-button and set rendering quality to low.
The second game I fire up every now and then is called Torchlight. If you know Diablo II then you can refer to it when I say that this hack and slash is just as fun, without the multi player part unfortunately. The single player mission is not that long but challenging enough, and the cartoon-ish graphic is easy to get used to. There is a button called 'netbook mode' which slaps the sliders to their far left position.
Older games are still fun! Why would you throw away your Age of Empires II CD, if you can put it into an external CD drive and play it on your netbook? Starcraft works perfectly, considering that the second part comes out soon I'm sure many of you dust off the first part to freshen the memories.
Half life and its modifications scored some world records for being most popular first person shooter of all time. Not too shabby in the light of the original Doom. Half-Life was released in 1998 with a suggested configuration of 133MHz and SVGA video card. Any netbook can do that, so playing Counter-Strike will be a no-issue. The source engine is much more demanding and the integrated video card may fall short in performance.
With some nuts and bolts script writing and tweaking you can get World of Warcraft to run on a netbook, but you really do not want to do anything serious in it without the netbook comes paired with Nvidia Ion. Games do work on netbooks but you have to either change settings to the bare minimum, or pick ones that are not good because of how they look.