I have just recently moved to Photoshop CS2 from Photoshop 7. The photomerge in CS2 is excellent for assembling mosaics (particularly from long lenses / scopes).
I do have registax and play with it a bit, but I'm far from knowing what I'm doing with it. I know what it's supposed to do, but working with just a few images at a time, I am yet to achieve what I expect it does... I think I just need to get with someone who knows what they are doing with it so I can learn a few things...
I also regularly use Arcsoft Panorama Maker. I take a lot of panoramic landscapes (and also industrial photos) and it does an excellent job even with wide angle lenses. It has a few limitations (e.g it won't even work with images outside certain proportions but if you need to assemble a mosaic it is possible by planning the sequence you join you images. For less than AUD$40 for a registered copy, I can't recommend it highly enough for panoramas, and it might make a viable low cost alternative to other software for assembling mosaics (if you are patient enough).
Al.
PS. I have also used Panavue as a trial version. It is more tolerant of conical distortion errors than Panoramam Maker, but produces some odd shaped images. I think it maintains angular position well so for astro work would be fine if you aren't worried about necessarily having a rectangular image when you're finished...
