In jaunty my wireless card would cause kernel panics while writing to ext3 (known issue, bug report filed). Also needed to use a custom repo/manually install nvidia drivers as the one provided by the OS were unstable to say it nicely, and a giant massive headache to say it not so nicely.
In karmic my wireless card will not connect to WPA encrypted networks (known issue, bug report filed, still no fix).
Lesson Learned: Wireless hardware support is still garbage even with mostly generic Intel wireless chips in linux.
That being said, otherwise I've enjoyed Karmic quite a bit and haven't had any of the other issues others are claiming. If only my wireless could work 100% I would be in love.