I've been using the GDW D20 system for many years as referee. We started with Twilight:2000 v2.2 and then continued with Traveller:The New Era. And right now, we are using this same system for Dark Conspiracy. In fact, we have some ongoing campaigns in the three games.
I would not say that the GDW D20 system is the best, but it's true that has allowed me and my players to switch between three game worlds we consider very attractive with very little effort. If I must choose one important point in favor of GDW D20 system, I will take this.
Now, just talking about how the rules work, I've played CT in the past, and I find the TNE system much more suitable. And this is especially true when talking about the combat rules. Once the players have used the system in a pair of game sessions, the combat is fast, furious and detailed. If I must choose a game system as a complete package, I will go for GURPS. But if considering only the combat chapter (focused in modern and sci-fi ranged combat), GDW D20 is more suitable.
Rolling one dice for shot (except for high rate of fire weapons, as Aramis stated) does not slow the game. If you have Small Rifle:12 and ten rounds can hit the target, it is very quicly to roll a handful of ten dices and see how many of them are equal o less than 3. Ok, I’m simplifying. There are location, damage, armor values, ranges, recoil, etc. But, from my point of view, the rules give the player more choices and are quite easy to use with some practice.
The system has some weak points, too. It is true that playing characters can suffer a severe punishment before falling dead. I think this is an old discussion matter among the users of the system. But injuries have consequences (initiative reduction, unconsciousness, the need of constitution rolls…) and players can have the feeling that their characters are in risk in every combat situation. A head injury (using the double damage rule if D20 is less or equal the damage suffred), can kill some characters and put others out of combat.
Difficulty levels could be another weak point. But they are easy to handle and you will never find yourself calculating strange numbers. The referee can considere to use small modifiers to the target number is fine adjustment is required
We use some house rules (variable Characteristics depending of how a skill is used, for example), but my overall opinion about the system is positive. It could be better, but works good enough.