With 5.6 being a required-fee upgrade from 5.5., i'd rather stay with the free 5.5.2 update that came out about the same time. 5.5.2 also adds Universal Binary to 5.5, and has almost all the new features except the HDR support.
5.5.2's smart blend module has finally reversed the trend of every new stitch/blend module being significantly slower than the last. On a 4-years-old 2x2 Ghz G5, not one of the intel macs, 5.5's smart blend would require about 8 hours to build a 37-shot spherical panorama at 18000x9000 pixels and 3 GB RAM, 5.5.2 completes the same task in about 2 hours (all other settings equal) and is now as fast as the last major version while having much better image quality.
So, for me, 5.6? Not anytime soon, but i'm not ruling it out once i've got some extra cash on hand.

While the interface may be un-mac-like, i'm even less happy with the other offerings in the panorama stitching market, so for me it's the lesser of several evils... at least the workflow couldn't be much simpler:

- drop images on empty window
- press "auto stitch"
- have a coffee (~ 2 minutes depending on amount of images and complexity of scenery)
- set render size
- have a break while the program does the heavy pixel pushing