Summary: Astrosynthesis (AS) is a good product for the basic user, but users who want to use its more advanced functions will be let down by a lack of support.
Review: From what I have seen on the NBOS forums, there are two types of user for AS: Basic users are those who just want software that allows them to make a 3D starmap at the click of a button, and then tweak the results to their liking. Advanced users are those who want to delve more deeply into the scripting and customisation side of the program, or import their own starmaps for their own settings into the program to tweak and develop. The basic users are most likely to just want to use the program a couple of times to generate their setting and then not really need it anymore, whereas the advanced users are more likely to be using the program for lengthier projects and will be making use of the scripting and advanced output settings.
The "Basic users" will find that this program is pretty decent. The 136 page manual is fairly complete and well-written and helpful in this regard, and the creation settings are reasonably customisable. However, once the universe is created then a fair bit of work may be required to tweak it to the user's needs (e.g. the author has a strange definition of "hospitable planet" which amounts to "an environment that won't instantly kill you" rather than a world that is remotely "hospitable" by any normal definition of the word), and some of the astronomical information in the glossary is not accurate. But overall if you are a basic user then you shouldn't have too many issues using the program if you read through the manual. It is likely however that basic users would have to do a fair bit of 'post processing' of the results in a spreadsheet program to get it to really match what they want for their settings.
Advanced users, however, will have a lot of problems in getting AS to do what they want it to do. I consider myself an "advanced user", having used AS3 to make realistic maps of the stars near Sol for my Stellar Mapping page at http://evildrganymede.net/rpgs/stellar-mapping/ , and to attempt to update the Near Star Map in 2300 AD ( http://evildrganymede.net/2012/02/13/stellar-mapping-2300ad-near-star-map/ ). The scripting functionality ("AstroScript") is potentially very powerful and can grant the ability to make custom plugins that let the user find distances between stars, create custom displays (e.g. different route for different empires), change the distance that labels are viewed at depending on world parameters, or even create a new method of world generation. Other more advanced functions are the ability to import stars from csv files, and output 2D star maps. Pretty much everything on my stellar mapping/2300AD pages has been created using the Astrosynthesis' more advanced features.
The problem is that there is little to no support available for these more advanced aspects of the program. Astroscript is a variation of VBScript, so if you have some experience with that (or basic programming) then you may be able to figure out how to write a script or plugin to get it to do what you want. However, the API documentation is pretty much useless. It is little more than a bare framework, with most of the terms and functions lacking any explanation or examples beyond what type of variable they are. Additionally, the NBOS forums are pretty useless for support on this matter since the program's author is not interested in explaining how to use functions or in filling in the gaps in the documentation. As such, the user is left to figure out for himself how to use these functions... and more often than not will be unsuccessful. The sad part is that there isn't enough of a userbase on the forums to help out other users either, because the poor API documentation and lack of "official" support drives most advanced users away.
It's unfortunate because Astrosynthesis is a program with a very specific purpose that has the potential to be really useful for those who want to create 3D starmaps - but the lack of support is likely to frustrate those who buy it for that purpose. The market for this program is pretty small, so one would think that NBOS would be interested in supporting the people who pay $35 for AS3 to use the advanced features that are advertised - but that support is not forthcoming (which is very disappointing given the cost of the product). As such, people who are interested in using the more advanced features of the program will currently have to be prepared to face an uphill struggle to get the program to do what they want it to do, and should be prepared for little help (and should be prepared for the possibility that despite their efforts they just won't be able to use the program for their purposes at all).
So is the program useful for advanced users? Yes... but only to an extent. The advanced user is likely to have to lower their expectations of what the program can give them though. Despite the great potential of this program, the lack of support forces me to give this product a low mark (2 stars out of 5) for advanced users. For Basic users I would probably rate this 3 stars out of 5.