What, Serenity is from next year?
Well, yeah, Firefly is a pretty good show, and it is a 'western'-style sci fi story. Clever writing, good dialogue, and likeable characters. I do recommend that one. Think of it as 'Han Solo as written by Joss Whedon.'. And yes, Farscape was also quite watchable and then some. I'll second that recommendation.
Stargate is a bit more iffy, the first series is rather outdated, but Atlantis and Universe is decent enough; I'm liking Universe quite a lot.
A recent sci-fi series at least worthy of a mention is 'V'. You might like it, and the old folks might have seen the series it's a remake of.
Trailer: (http://www.youtube.com/watch?v=ahjPQjQGdbU). (I didn't like it but it did get a second season, it may have picked up after I stopped watching.)
As for other science fiction series.. an animated one, but not at all childish, is Ghost in the Shell: Stand Alone Complex.
Intro: (http://www.youtube.com/watch?v=rUk2r-cJtTs&playnext=1&list=PL15C1A552829C3E01&index=1). Very heavy, philosophical, and at times quite strong. One of the best series I've seen.
Star Wars: The Clone Wars (recent, not the old one) is an animated series based on Star Wars, and should appeal to both a young audience and older. I thought it was better than movies II and III, for what it's worth, but unlike Ghost in the Shell, it is targeted at a young audience.
If you don't need the series to be SCIENCE fiction, but just fantastic in some manner, I would strongly recommend The Walking Dead, which is probably the best new series of last year. True Blood is also well worth watching, as is Heroes. First season was the best one, IMHO, but still.