3G3 is overall a good suppliment. It's basically a really long physics model, converting it into a generic system (the infinitely and inanely complicated Time Lords system) which can then be converted into other systems.
Frighteningly, converting from the Time Lords system is the easiest part of the book.
It is nice for being able to produce new and interesting firearms (or recreate actual firearms if they're not emulated in your game), as well as rockets and missiles (and gyrojets, in fact), railguns, lasers, and particle beams. It also has systems for melee and muscle-powered ranged weapons.
It is a bit complicated (get good with Excel, 'cause otherwise it'll be painful). It also requires an in-depth understanding of your own game setting (the technological levels have to be really precicely detailed). Be prepared for math far more complicated than most algebra problems you find in gaming (for instance, there's a logorithm involved in converting damage values from the core system to Cyberpunk 2020).
This is not a suppliment for players, or even for GM's satisfied with running stuff from the book and not adding significant amounts to a game (adding a gun or two will not make this book worth it). It is for GM's who like in-depth detail and are willing to put a lot of effort into their game. More than likely, it's for GM's who are writing their own game settings.
[4 of 5 Stars!]