the reality is that isis don't need to send people over here because there are enough people here who are open to being radicalised already.
Also, isis coming here is a different animal to what they are doing in Syria. If isis want to be here to commit acts of terror it is easy enough to do as recent events in France, Australia, Tunisia demonstrate. But isis coming here isn't going to mean rounding up Christians to behead them on mass etc as happens in Syria, Iraq etc purely because the infrastructure here is so much different to those parts of the middle east.
However, it is naive to think that everyone fleeing Syria are lovely people just wanting to be an upstanding member of a society. Fact is that if a million people enter Germany for instance, it is inevitable that some of those will be undesirable, will have a past of some sort, or will be capable of committing any number of hideous non-terror-related crimes in their new countries of residence.
That doesn't necessarily mean that we should start turning away refugeses, however it does IMO mean that people should think more carefully before campaigning for free movement of refugees across Europe, because once you open borders, knowing just who you are letting into your country, and in some instances your home becomes impossible to know.