I've been using the CS4 panorama merger, and have been impressed. I don't have experience with other panorama software other than the entry level Canon PhotoStitcher, but the CS4 implementation seems very good, if not excellent. It not only is more or less seamless, but seems to be able to come up with a very good to excellent gradient to avoid visible differences in exposure.
With 10 or less files from a Canon 50D DSLR (15mp), the CS4 implementation of having Bridge invoke PS works in a relatively timely manner. They get done within several minutes on a relatively modern Windows computer (Intel e8400 core-2-dual 3ghz with 8gb of DRAM).
However, I've gotten interested in "giga-pans" with a billiion pixels (about 150 overlapped images from the 50D). These take forever. I reduce the original files from about 4800x3200 to 2700x1800 (about 1/3rd of the number of pixels), I tackled a panorama with about 200 images, and it took about 16 hours to process, even with the reduced resolution. And when it got done, I could see some short-comings from curvature, and some gaps of coverage.
It would be Very Helpful to get this information back in an hour or less from a "draft mode", rather than taking 16 hours to do an excellent job.