First, "respected sci-fi" is a contradiction in terms. "Sci-fi" indicates something more shallow, meant just to entertain.
When Star Trek is done well, it's pure science-fiction. Season one of the original is the one time that solid, great, adult science-fiction was done consistently for an entire season, except possibly Outer Limits and year one of Space:1999. I've been marvelling lately how smart original season one Trek was, and how much thinking the viewer is called upon to do (as opposed to Next Gen where the simple, obvious moral is pounded into you). These episodes are like films, and far, far better films than those maudlin Hollywood effects ST movies of the 80s.
Recently I watched some of the best original Trek, then Wrath of Khan. The episodes were like nourishment for the brain. Khan (despite being the "best" of those movies) was utterly empty, with simple cliched lines and a ranting villain, and I felt as if my brain was being starved.
If an SF premise or question is at the core of the story, then it's science-fiction. Original Trek, year one anyway, did that. Star Trek wasn't about itself then, fixating on its own characters and their careers, but was about SF concepts. By contrast the films often had little or nothing in the way of an SF premise... just bad jokes, melodrama, and moments stolen from TV episodes.
My Star Trek is that first season. Every other season of every other series I judge by how well they live up to the depth and intelligence of that season. Seasons and series that don't, I don't think of as Star Trek. So, yes, for me, Star Trek is just about as "science-fiction" as anything can be. When later "Trek" degenerates into mere space adventure, it's a betrayal, and not what Roddenberry, Coon, etc., were working hard to get onscreen. It's Hollywood product.
Now, is ST respected by literary SF fans? Hang out at a convention, and you'll hear respect and disrespect. Plenty of each I think. Remember, a lot of SF in print is extremely "soft" SF, without real SF concepts.
