Just recently acquired this. Not really a review, but wanted to note the quality is up orders of magnitude from the original Prime Directive d20. The original version seemed to be an awkward conversion of GURPS into d20 - skill packages, races that didn't quite follow d20 conventions (i..e odd ability score modifiers, etc.)
There's still a few wrinkles (I can't imagine why anyone would want to play a human - Alpha Centurans seem to have all the advantages and more) but it a far, far more solid system than its predecessor. If you're looking for a d20 version of Prime Directive (or some series of tv shows and movies that seem similar to it...) you wouldn't be disappointed.