We used Traveller (in this case I mean Classic and Mega Traveller) for some space-operish campaigns, though the combat has a bit of a gritty edge to it. It worked out anyways.
Weapons tend to be a bit more realistic than in most space opera. No technobabble weapons like "phasers" or "blasters", and it is assumed that energy weapons, while they begin to be available fairly early, need some pretty big power supplies. That said, some of them do some pretty hideous damage at higher tech levels.
I rather like T20. The class arrangement is a lot more sensible than most hacks I have seen at them in d20 SF games. However, the chargen takes some getting used to. Here's a tip if you buy it and have trouble with it: class determines what profession you can enter, not vice versa. Think of class as the skills you list on your resume, and profession as the job you are trying to get.
I am considering tinkering together a master-of-orion game using t20 rules, or a similar setting with lots of alien diplomacy and warfare.
Space travel: I am a rabid traveller fan, but there is one thing that was always a conceptual hurdle for me: 2d spacemaps. I mean they worked great for player autonomity, allowing players to decide where they go next. But conceptually, it just doesn't work for me.
FTL travel is cool. There is no "subspace radio", which means that courier ships must carry interstellar messages, and there are a lot of surprises in space combat.
Jumps always take one week, regardless of distance. This is good to give the players some down time/training time/healing time, and is also great to isolate the crew of a ship for a horror or mystery scenario.
If you want to go the non-d20 route, fortunately you can still get classic Traveller reprints. MegaTraveller I feel was a better, more brushed up version of classic, but it is long out of print, but no reprints, so you'll have to get it used if at all. Avoid TNE and T4 like the plague. If you go with the reprints, at least try to scrape up a MT player's manual or referee manual so you can at least have the nicely codified task system (vice the rather ad hoc one that existed in CT.)
If you like GURPS, GURPS Traveller is currently supported. Personally, though, I dislike GURPS and think that it does not capture the feel of Traveller nearly as well as d20 does.
One nice thing about T20: World and ship statistics are printed in the same format as CT/MT, so if you get the reprint or old products, you can plug them in to T20; the only thing you will have to convert is characters and creatures.