I just bought/downloaded The Core application for
Starry Night Digital Download 6 (around 100mb's) and I must say that for around $25US it's a bargain.
I had an old old version of Starry Night Backyard but it had only limited objects in its database. I've had/tried other programs like Stellaruim etc and while they are great/free programs they don't seem to show up to date things like comets/asteroids etc.
There are additional addons/data modules available for download for more $$ that are
optional extras. From what I can tell you don't need them to enjoy and use the program properly.
I've only just started playing around with it but I'm very happy so far.
I've attached an actual picture of the screen display from my location @ approximately 11pm, 10/11/07 showing comet Holmes position in Perseus among other things.