Short- and medium-term, anything that leads away from fossil fuel is a good thing. Long-term, though, it seems like taking energy that would normally have bypassed the Earth, capturing it, and then adding it into our ecosystem is probably a bad idea. I'm a total layman, here, though, mostly going off of what I've read in science fiction.

Also, yeah, the weapons potential is a problem, though I sort of feel like we've already got all the weapons we need to intentionally eliminate human life, so further refinements don't matter much. It's the accidental extermination we're worried about, and this might help with that for awhile.