It's exciting to think that we might have all kinds of critters living in places (is it Ganymede?) where there may be liquid water (near such a vent) that's otherwise cold and far from the sun (or a sun).
The million-dollar question for me has to do with the origin of the extremophiles. We don't really know if our earthly critters formed in all of these hostile environments or if they formed in one environment and then radiated out to fill other niches, evolving to fit. You could see it going either way. If life can form anyplace that's remotely habitable, then we might have reasonable expectations of finding it on Mars or Europa or Ganymede. But if it has to form in some more pleasant place and then work its way into extreme environments, we'd be less likely to find something in places that never had Darwin's 'warm little pond'.