From a technology standpoint, I think it is, at least the way it's presented in ME1. As I pointed out in another thread, the realism of the series continued to weaken with each installment, but ME1, a lot of what is offered is, I think, a pretty realistic take on things.
Their explanations for FTL, Element Zero and its effects of increasing or decreasing mass, the way the weapons functioned, and Mass Relays all have some foundation in real-life science, either practical or theoretical.
Even the Omni-tool isn't that far-fetched, in fact, there are some companies and universities working on concepts that could be viewed as precursors to something like an Omni-tool.
If you think about it, just look at how far we've come in the past 50 years or so. Then multiply that 3 or 4 times for the 150-200 years in the projected future that Mass Effect takes place, and I think it's pretty plausible.
As ZipZap pointed out, the alien culture, politics, and human-interaction with those is all up for debate. I think that could go just about anyway you could think of, so I think that part is the most open to a person's interpretation as to what they think would happen. I think Mass Effect is an awesome vision of what could be, as far as any of that goes.
As for the reapers, I don't know. They were a contrivance for the sake of the plot, which is fine. They were both a blessing and a curse in that regard. I never liked the idea of the reapers much because the whole Lovecraft thing is overdone I think, but for what they were they were fine I guess. Whether they could actually exist, who the hell knows. I think that was the entire point of them, in a way. They were this great inconceivable horror of a threat, but once again, the Lovecraft thing never excited me, but Bioware are clearly Lovecraft fans, and that's fine.
So to sum up, technologically: yes. Societally and culturally: maybe. Reapers: anyone's guess. My guess is, I doubt it.