Overall this extension is great. The only negative I could find was the rendering performance under Firefox 2 (not sure about 3 yet, which uses a new rendering engine). This heavily noticeable when scrolling up and down. I know it's the fault of this extension since I tried disabling it and immediately the rendering performance was great. The rendering performance is also great with IE 6.0 (haven't tried IE 7 yet).
I don't know if it's possible to fix whatever is causing performance problems, but this is the only problem keeping me from giving it an excellent rating.