I tried PTGui but found the interface to be pretty crude and not very Mac-like. In fact, it is a cross-platform application, however simple Mac things like drag-and-drop don't work the way they should on MacOSX.
I don't have Photoshop - it's possible that it has a photo-stitching mode, but at whatever-hundreds-of-dollars it costs, I'll never know.
I wanted something that would work well with iPhoto.
I ended up purchasing DoubleTake which is very easy to use, accepts drag-and-dropped photos from iPhoto, and costs $24.95. I posted my first panorama photo on flickr. The results for a very quick-and-dirty run look really good. With some work I can warp the images to fit better (where the lower edge of the car don't meet well in the merged image).