The base is good for the United States, obviously - It gains another installation around the globe to combat potential Iranian threats toward its interests, and it expands its influence in Eastern Europe against Russia. I'm not so sure it's the best thing for Poland, although I don't believe it is the catastrophe that some are making it out to be.
My knowledge on the subject is limited but I have felt EU as one of the best models of democratic union resulting a very good deal of good.
If forced to choose between strengthening ties within the NATO alliance or strengthening ties with Brussels, I should surely hope that Warsaw chooses the former, as the EU is an independent nation and culture's worst nightmare. NATO may attempt to take advantage of Poland for U.S. and British military interests, but it won't strangle Polish culture and do to Poland what the EU has done to Germany, France, and is trying to do to the United Kingdom.
Come sisters and brothers of Poland, come join in alliance with Serbia, Ukraine, Byelorussia and Russia. Then, Czeska, Slovakia, Bulgaria, etc, etc.
We Slavs have our bussines, we have bright future
Russia tried fighting for Pan-Slavism in World War I; it cost them the Romanov dynasty and the freedom of their nation to a scheming pack of Bolsheviks. Bulgaria sided against Serbia in the Second Balkan War, in World War I, and in World War II(sided against Poland as well, I might add). The Czechs and the Slovaks split for a reason.
Slavs have a bright future, yes. We also have many differences, and no Slavic nation has hesitated to side with the English, or the Germans, or the Americans, if it so benefitted them.