Are all decoders(skybox etc) the same?

I have a couple of spare decoders at home and was wondering are they for the most part the same. As a sim card can be used in any (unlocked) cell phone, can a viewing card; be it sky, virgin or even a european one, be used in any decoder. Are they region specific, country specific or are they completely mix & match. On a recent trip to Poland I bought a complete Polsat digital satellite package and I'm now thinking maybe just the viewing card and the bigger dish would have been all I needed. Anyone got an informed answer on this one?