Okay, I'll spread my opinion here. I am not great with d20, but my buddies looked over the core rules:
essential, good rules, sound meachanics, suits the game (the rules that is)
There is reams of stuff in the core book, and all you need IMHO. After all, Signs and portents articles are free.
The Cold Equations is a sound scenario.
I like the complete campaign, the ragged edge, which I have, but I feel, now no offence to the author, that it does not really capture the feel of babylon 5, but is perfectly functional, well written, and makes minor references to other games (names like Lustria and ORC).
I also have heroes and aliens. A lot of profiles for sheirdan, the characters of the series, mordon, bester, crusade characters and aliens. Aliens a bit disappointing. You have a lot of options, but again I feel some of the descriptions for the races does not make much sense, well, I find them not being true to Babylon 5. Some are real good, like info on the hurr and llort, others kind of odd, at least background wise. The rules here are sounds, but some of the back story is a bit dodgey (no offence). I just don' think its right. Good stuff on the Dilgar, but some of the races are confusing. As there is only a picture of some, I have not a clue what half the races actually are. Also, a very short book for the price, but very useful nonetheless.
GM Screen,
Lovely, I mean gorgeous. Buy it for goodness sake. Even if you change your mind about the game, just get the screen.
I would not touch any first edition books, because you'd have to root around for the rules conversions. Despite my criticisms, the main problems I have are easily fixed at GM discression, and I regret none of my purchaces.
When I have time, I will get: ship plans (essential I think), legend of the rangers (essential for ranger campaign, but there are rules for rangers in the core book), the drahk book looks good, the rim sounds cool, and the thirdspace book is something on my priority list.
I hope that helps. All in all I like it but have reservations about certain things.