Been a disaster in the past. With 2.5 i had better luck.
Past versions would often insert artwork for entirely different artists. Haven't had much trouble with that using 2.5.
However, not all is well. It inserts the art into all songs of the same album, even if only part of the tracks are selected. It sometimes reports that all art has been found, when actually it has made no changes. It also is spotty about putting the URL into the comment when that feature is active.
Worst, often when track, artist, and album are all present in the tag, iTunesCool will still insert artwork for the correct artist but wrong album.
It seems to at least get the correct art as often as FetchArt, (about 60% of the time) but still has it's dreadful text-only interface.