Well in general the Slovaks are much more friendlier than the Czechs. That being said, a little command of either languages will be of immense use and the czechs/slovaks very much appreciate that (as they know that their language/s is/are not in many countries).

The problem arises when the Americans/Canadians/British start comparing the facilities and the cost of things (as their currency is stronger). The czechs really don't like that. But in general, they're reserved (although they can be friendly if you attempt to speak a czech phrase or two).