So far, several sites have posted previews, sample images and measured performance on the 7D. They all carry the disclaimer of a 'beta camera' and promises an even better performance when the final version is released.
Here are my thoughts so far:
1) High ISO Noise - As far as I am concerned, I only use up to ISO 1600. ISO 3200 is a nice to have but I will only use it in extreme emergency. From the sample images, ISO 1600 is definitely better than 50D. Some sites says it is about 1/2 a stop better than 50D and that it is now a draw with the D300. In my opinion, it is still a little behind the D90. However, taken into consideration the 50% can in pixel count over D90, I think the high ISO performance is simply stellar.
2) AF speed - The measured full AF speed by imaging-resource put it at almost double that of the Nikon D300. While pre-focused, manual focus and contrast detect focus speed is behind the D300, it does show Canon's new AF system hold tremendous potential
3) Resolution - From my visual observation, the 18mpx seems to have been given a slightly lighter low pass filter, giving it a good resolving power. I have yet to see a comprehensive MTF chart on this but with 50% more pixel, I doubt Nikon can pull any more tricks to match the resolution of the sensor. (Nikon managed to somehow match resolution of the Canon 15mpx 500D with the 12mpx Nikon D5000 when ACR is used to convert the raw files.)
Other unmeasured aspects include:
1) Dynamic Range - This should be interesting to see.
2) True MTF Resolution
3) AF Tracking accuracy and speed
Other aspects of the camera which cannot be truly appreciated by just reading include:
2) LCD screen
3) Feel of the grip
4) Overall responsiveness