I was updating my cover art last night and had some problems.
In MJ8 when you downloaded cover art you were presented with the image and asked if you want to use it or a different one on your local hard disk.
In MC9 you aren't given any choice. A cover art image is either associated with the album or not. If no image is found there doesn't appear to be a way to you used to be able to point a track or album at a specific image file like you could in MJ8. Also you can't edit the cover art tag manually to correct this. I found it a real pain to have to download an image, save it to my cover art folder then give it the same name as the album so that MC9 would pick it up.
I also had problems with compilation CD's. For example, I have a Fleetwood Mac box set with 6 CD's. I want to use the same cover image for each one. The albums are called "The Complete Blue Horizon Sessions (CD#)" where # = 1 to 6. I had an image file called Fleetwood Mac - The Complete Blue Horizon Sessions (CD1).jpg which was associated with the first album. None of the other 5 albums would pick up the same image even with the low tolerance option selected and I forced it to look at the local hard disk only.
The only way I could get this to work was to have 6 copies of the same image with the same file name as each album. In MJ8 I was able to have just 1 image because I could explicitly point the album at an image.