I think in a discussion like this, it's important to distinguish between "good" and "entertaining." Often the two overlap, but you may still watch a movie that you don't particularly ENJOY, but which you recognize is well-constructed.
So, why is Blade Runner "good"?
Well, visually, it's stunning. I mean, even if you find the film boring, you can't deny that it's impressive to look at. Syd Mead's design, the art direction, all of it. It's visually very impressive -- doubly so, considering it was made in 1982 and (in my opinion) still holds up visually. In many ways, it also set the standard for cyberpunk and visions of an urban dystopian future where you DON'T have some post-apocalyptic wasteland. So, it's "good" in that sense.
The story itself is also, in my opinion, interesting as a concept, and the film at least TRIES to really pose and struggle with questions of humanity, identity, emotion, etc. It's high-concept sci-fi. You may still find it boring, but you can't deny that at least it isn't just "I dunno. Some movie about a dude who kills robots and stuff. Oh, and it has flying cars." It's ambitious and tries to be the best that sci-fi can be. It filters fundamental questions of humanity through the lens of the fantastical to help us explore those questions, and even if you don't find it particularly ENTERTAINING, I'd bet most folks would still say "Yeah, but it's not just big dumb action sci-fi."
To me, those are important reasons why it's "good" as a film. Citizen Kane has been referenced a few times in this thread. Even if the subject matter doesn't grab you, even if you find the presentation of it BORING, you can't deny that the film is well MADE, and is important in terms of film history and the development of modern cinema. I tend to think that, within science fiction as a genre, Blade Runner is....close to the genre's Citizen Kane.
Blade Runner was a big-budget high-concept science fiction film. It differed from the cheesy low-budget stuff that had come before, and it wasn't a big-budget excuse to showcase robots and flying cars. See, highbrow sci-fi -- sci-fi at its best, in my opinion -- filters important questions through the lens of the fantastical. It explores concepts by placing them in a fantastical setting. The setting is the vehicle for the conceptual exploration. Lowbrow sci-fi, by contrast, takes the setting and makes THE SETTING ITSELF the point. In that case, the story is just an excuse to have robots and flying cars show up. Lowbrow sci-fi can be plenty entertaining, but it's essentially fluff. It doesn't really do any heavy lifting.
Ultimately, the same is true of any genre product. You can have "lowbrow" horror, like, say, Resident Evil or Return of the Living Dead. Or you can take the SETTING of the zombie apocalypse, and use it as a vehicle to explore human nature, as with The Walking Dead. With one, the setting is an excuse to have zombies eat people and get shot in the head. With the other, the setting is a method by which you watch how people respond to an apocalyptic scenario, and explore their human nature (particularly in contrast to the mindless, reflexive zombies). One's an excuse to showcase Tom Savini's F/X work. The other's a vehicle to watch, say, a character slowly go insane from the pressures of the harsh new world in which they live, especially after witnessing so much death.
As for whether any of this stuff is ENTERTAINING, well, that's a matter of viewer taste.
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