This is a comprehensive and well executed adaption of D20 to Traveller. Traveller is a game I've held in high esteem since I was a young teen in 1983. The other reviewers do a great job of describing what's appealing about Traveller, so for the sake of brevity: ditto.
But not only that. It's a better D20. Here are the areas where I think T20 does better than D20:
1. Armour exists not only to deflect weapons, but to reduce damage.
2. Being more experienced doesn't make weapons less damaging. A sword or pistol is always a dangerous thing to have pointed at you.
3. People who work together have different backgrounds, different levels of experience, and different ages.
4. Magic Powers (in this case, psionics) are built on the existing skill system and bought with skill points.
5. In many cases, you can compensate for poor education by having high IQ, poor charisma by higher social standing, etc.
6. Experience points for completing objectives, rather than winning fights.
If you like lots of combats where your character takes a half-dozen 10-point sword wounds in a row and comes out fighting, and you get masses of XP for this, then this game isn't for you.
If you like a lot of variety with exploration, role-playing, a few *very* dangerous fights, space ships, high-tech, low-tech, utopias, hell-worlds, commerce, aliens, etc, then try this game.