I'm on holidays in Poland right now, and that installation is a pretty big thing, but Russians threats well, in last 20 years they always threaten everyone in the area. The most common one is that they simply gonna cut the supplies of gas and oil, but i think we just got used to that.
Western public sees that installation as an answer to Georgia situation, when in reality plans for that system were created in middle of 1990's, and proposed officiali in 2006. Russian elites still believe that they own Central Europe, that this is their sphere of influence, sadly for them it's not anymore.
Russia cannot be ignored, because it is still an empire, controlled by FSB (Former KGB and NKWD) under quasi democratic banner. But at the moment they would be unable to do anything, in the future maybe? Only danger that countries like POland face is energetic crisis, when Gazprom asks for too much for their natural resources.
One more thing, how Purely defensive system can be controversial? this cannot be aimed at russia because the only task of those missles is to intercept rockets aimed at US soil. And poland in exchange is getting Patriot missle systems (which are also a defensive unit)
