They are usually considered allies in the context of WW2. The Von Ribbentrop-Molotov pact was evil, though, and it doesn't look like the USSR would have attacked Nazi-Germany if they hadn't invaded. So in that sense, it was an ally only out of necessity.

IMO the discussion often boils down to definition of words and emotional attachment to those words. I don't think people here disagree on the (most prominent) facts.