For the most part I absolutely adore this game. The setting is one of my favorites (the "Stargate SG-1" TV show universe is dead-on perfect as an RPG setting). The system (adapted d20; you'll need the "Dungeons & Dragons Player's Handbook, third edition) is wonderful--in particular it's stunningly detailed, allows characters to try virtually anything, and is quite flexible. It's simple enough to understand without much work, but complex enough to allow for dramatic, varied game-play. The amount of information provided is fantastic.
The one down-side of this much information is that we keep forgetting that there are rules we should be applying to the game because there are just so darn many of them. Over time this abates--you get better at remembering what's in the book the more you play. Run a few combats before you play the game, so you aren't trying to learn both at the same time.
My real gripe, and the one that took away that coveted last star, is that the editing job is terrible, and the layout job with respect to the charts doesn't help. The editing problems are issues that noticeably interfere with game-play. Paragraphs trail off in the middle. Fractions have been replaced by letters (just how much can you blow up with 'G' lbs. of C4?). Charts are often found quite a few pages away from any explanatory text.
There are the usual few details that feel a bit wrong--the idea that characters should gain roughly one level of experience for each mission would have had SG-1 retiring at the end of season 1 of the TV show. The idea that any officer player characters have Major as a minimum rank doesn't match up with either the TV show or the info in the first half of this book.
However, these are minor issues and easily fixed. The company's web site ([...]) provides detailed errata in the forums, as well as additional resources. And other than these problems, this is still the most fun I've ever had either playing OR gamemastering an RPG. It's an expensive book, but it's also nearly 500 pages--not a single bit of it wasted in my opinion. So I highly recommend it; just make sure you look up those errata!